Precepts of 5e style are that spells do what they say, and don't do things that they don't say.
Precepts for 5e rules are that specific rules beat general rules, and that were nothing is mentioned the general rule applies.
The current, most up to date listings we have read like this:
Darkness: "Darkness creates a heavily obscured area. Characters face darkness outdoors at night (even most moonlit nights), within the confines of an unlit dungeon or a subterranean vault, or in an area of magical darkness."
Darkvision: "Many creatures in fantasy gaming worlds, especially those that dwell underground, have darkvision. Within a specified range, a creature with darkvision can see in darkness as if the darkness were dim light, so areas of darkness are only lightly obscured as far as that creature is concerned. However, the creature can't discern color in darkness, only shades of gray."
Lightly Obscured and Heavily Obscured: "A given area might be lightly or heavily obscured.
In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight.
A heavily obscured area--such as darkness, opaque fog, or dense foliage--blocks vision entirely. A creature effectively suffers from the blinded condition when trying to see something in that area."
Darkness (Spell): "Magical darkness spreads from a point you choose within range to fill a 15-foot-radius sphere for the duration. The darkness spreads around corners. A creature with darkvision can't see through this darkness, and nonmagical light can't illuminate it. [...] "
I'll reference these where appropriate.
The sentence in the spell says that creatures with darkvision cannot see through the darkness. This is worded this way because it means something very specific - and it doesn't mean or imply anything outside of that. This is a matter of 5e style. Things being worded in a particular ways are specific, and they mean specific things, even if we as english-speakers might normally use different language to express a similar point.
It says "creatures with darkvision cannot see through this darkness". The question I'd like you to think about, in this situation is, why does it not say "Creatures cannot see through this darkness" Or "Creatures that rely on sight cannot see through this darkness".
The reason here is that this sentence is Only applying, Specifically to creatures with the darkvision trait. This sentence applies, expressly, and absolutely, to No One Else.
What, then, does it mean? What could it mean that would apply all and only to creatures with the darkvision trait, and not all creatures, or all creatures who rely on sight? It means, and can Only mean, that the sentence is making a 'specific' ruling in relation to the functioning of the darkvision trait, to overrule the 'general' ruling for darkvision.
Normally, the 'General' rule in question here, is that characters with the darkvision trait can, if trying to see something in an area of darkness that exists within their darkvision radius, can see it as though it were in dim light, rather than darkness; this equates to lightly obscured, instead of heavily obscured.
The specific rule that the spell is making with that sentence is applying to that trait - it is saying that if a creature with darkvision is trying to see something in the darkness created by this spell, they still have to treat it as darkness, not as dim light, even if it is inside their darkvision range.
The intent here is clear: It is saying all and only that the feature of darkvision that allows a creature with it to treat darkness as dim light does not work on the darkness created by the spell, and as a result, even a creature with darkvision is effectively blind when trying to see something within the area of darkness.
In layman's terms, what this sentence says, specifically, is "If you have darkvision, it doesn't work for this darkness". That's all it's saying - it is saying nothing else.
Indeed, to say that creatures with darkvision are impaired here, but creatures with normal vision aren't would be absurd. If you want to say that that sentence implies actual impairment for creatures with darkvision, beyond simply not having their darkvision function, then that is the absurd conclusion you are necessarily arriving at - because the sentence only mentions creatures with darkvision, and mentions them specifically, to the exclusion of any more general grouping or category. Since that conclusion is, indeed, absurd... the only recourse is to realise that the sentence is not, and cannot be, implying any kind of impairment beyond the darkvision trait itself not functioning.
To say that that one sentence is ruling multiple things, and that saying "a creature with darkvision" is also intending to actively imply detriment to all sight-using creatures, including those without darkvision, feels deeply disingenuous to me: if that was what they wanted to say, this is, very obviously, not how they would say it... it's a very odd way of saying that the area "is opaque to creatures that rely on sight"... because that is what it would be meaning if it worked as you've said, and that is how it would be worded if it meant that; we have precedent for it.
Light penetrates heavily obscured areas just fine. There are a couple of cases where it doesn't, and those are explicitly called out. There is a difference between light passing through or penetrating an area, and light illuminating it.
"Light does not illuminate the area": this means a very specific thing, and it does not mean anything outside of that, not matter how rational it seems to you: it means that light cannot turn the area of darkness into dim light or bright light. A torch illuminates and area around it – bright light for a distance, and then dim light for a further distance beyond that. It cannot illuminate this darkness, even if it is brought into that range: it is darkness, and it remains darkness. This doesn't mean that it is opaque - again, if it did, it would say so. It means that it is darkness, nothing more, nothing less. Light passes through darkness just fine - that's just the laws of physics. It is a different thing entirely to suppose that an area of darkness is illuminated by light, or brightened up by it.
Light passing through darkness is what allows us to see the distant torch of a friend 200 feet away, while that friend cannot see us in the dark. Their light illuminates the darkness in a radius around them, but the light itself passes through and travels much further, allowing things in the darkness to see it. This magical darkness cannot be illuminated by natural light. The spell does not say anything about it magically preventing light from passing through it. If it did, the spell would say so.
If the spell was opaque, and prevent light from passing through it at all, that would have a significant impact on the spell - so you can rest absolutely assured that if the spell did that, it would absolutely, one hundred percent, say that it did. The fact that it does not is itself a ruling. As it is, it is just darkness, with a couple of explicitly defined magical features; the spell marks it as magical, and then explicitly defines the ways in which it is magical and differs from mundane darkness... but it is just darkness, beyond those very specific things. Light passes through darkness perfectly well without illuminating the dark space. We wouldn't be able to see the stars if it didn't. It doesn't illuminate the space in between - but we can see through the darkness to the stars that are illuminated. There is no corridor of illumination between us and them - just unilluminted darkness, through which light passes.
Presuming that the spell is opaque and prevents light passing through is, in fact, a massive, massive assumption, considering the impact on the function of the spell that such a detail would have. If it did that, the spell would definitely say so.
We have an example of a spell (Hunger of Hadar) that creates darkness that is opaque and does block light from penetrating or escaping: the spell says that it is and does. The spell also extends the blinded condition to any creature within its area – it explicitly calls out the fact that it does so.
Pointedly, Darkness does not say those things, and does not extend that condition, by its own wording. So I would question, given that we have an example of a spell that does this, and calls it out specifically, and we have an example of a spell that does not say these things and does not mention them, why do you feel that it is a valid interpretation to just go ahead and pile those details onto the spell that does not say it?
Saying that the Darkness spell is opaque and doesn't allow light to pass through it - saying that creatures inside the area of the spell are blinded - would be the same as saying that Scorching Ray ignites flammable objects... because burning hands does, so obviously scorching ray does too... despite the fact that burning hands explicitly notes that it ignites things, while scorching ray equally explicitly does not say that. The reality is that one spell ignites flammable objects and the other does not. We can rationalise or explain that however we like - the rule is clear. The same is true of Darkness: it does what it says it does. It does not do anything else.
There are effects that create opaque spaces, and effects that create heavily obscured spaces which inflict Blindness on any target within them. Those are something that we have, and which the game rules define. This is the proof that, when such tangible, game mechanical effects occur, the game notes them. We have that benchmark presented to us; in this case, then, if that note isn't given, then it's not intended to occur - if it was, it would say, and we have evidence proof to show that that is, indeed, the case - because there are places where it specifically does note it and call it out. To apply those riding effects and mechanics to spells and abilities that do not state them - when we have evidence of spells and abilities which Do state them to compare to, is absolutely house-ruling, and is absolutely not following the design intent of the spells you are modifying.
When something IS opaque, the game rules call it out: Tiny Hut, Wall of Fire and Arcane Gate, for some examples. Interestingly, Wall of Ice, the player spell is NOT opaque, while the ice devil summon wall of ice IS opaque - this is a difference that is called out specifically. Darkness does Not call this out. It is Not opaque. If it were, it would be mentioned. We have evidence that, if it were it would be mentioned, because other spells mention this detail where it occurs. You can rule that Darkness is opaque, but that is a house-rule that you are making, and it is not in line with the base ruling for the spell.
When an area inflicts the Blind condition on creatures within it, the game rules call it out: Hunger of Hadar is the best example already cited here. Wall of Sand is another. Darkness does Not call this out. It does Not inflict Blinded on creatures within it. If it did, it would be mentioned. We have evidence that, if it were it would be mentioned, because other spells mention this detail where it occurs. You can rule that darkness inflicts Blinded on creatures within it, but that is a house-rule that you are making, and it is not in line with the base ruling for the spell.
It is not common sense, that the spell does these things: if it did these things it would say so. It doesn't logically follow that the spell does these things: if it did, it would say so, just as we have other spells that say so.
Perhaps a person cannot logically see how a single character can make ten melee attacks in 6 seconds, moving ten feet between each one... so they say that, by common sense, you can only make a maximum of 4 attacks, even if you have other extending features. Someone can do that - but they most certainly cannot claim to be following the rules, and just applying common sense.
You don't need to dip into grey space of uncertainty for something like Darkness, because all the features of it are quite sufficiently defined. Each of the other effects that you and others have wanted to ascribe to the spell, which it does not say, are all things that other spells do, explicitly, call out and notify of when they occur. Pointedly, Darkness does not do those things.
Tiny Hut does not spontaneously generate food and water for the party "Because logically, food and water are required for a comfortable living condition as the spell says it creates"; no, if it did that, it would say it did that.
Darkness is not opaque; darkness does not inflict the blinded condition on targets inside it. There is nothing in the spell that suggests it does these things, at all. It's pure house-rule to suppose that it does.
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What it is, is 'darkness', which cannot be illuminated (Changed from darkness into dim or bright light) by anything short of stronger magic, and for which mundane senses, such as darkvision, do not function. It can also be moved around by being anchored to an object. Beyond that it is JUST darkness – no different from regular darkness except in those very specific ways which the spell defines.
Regular darkness is a “Heavily Obscured” area, the general rules for which are clear. It is functionally, for mechanical purposes, identical to dense foliage, to use the example cited by the rules.
Targeting in 5e is always about what you are trying to see, never about where you are. Read it above; you are treated as blind when trying to see something that is in a heavily obscured area. It makes no mention of where you are. It's about what you are trying to see.
If you are a ranger, hiding in a patch of dense foliage beside a path, aiming to ambush a merchant, you are in an area that is heavily obscured: the Merchant cannot see You. The merchant is in the open and brightly lit. You can see the Merchant just fine... If this were not the case, you couldn't ambush them at all, ever. If being in an area that was heavily obscured impaired you from targeting out of it, there would never be any ambushes... the ranger would be at disadvantage to attack the merchant because of their own hiding spot (though if they were also unseen, they'd cancel it out, of course). This is ridiculous and obviously not the case.
If you are in darkness, you are in an area that is heavily obscured; creatures are functionally blind when trying to see you. You are not, yourself, blind. If your target is Not in an area that is heavily obscured, and is, in fact, well lit, then you do not have any impairment to attacking them. If you are indeed unseen by them, then you'll have advantage.
If your target is also in the dark, and you are both in darkness together, then you are both trying to see a creature that is in an area that is heavily obscured – you would both have disadvantage; you would also both be unseen by the other, and so it would cancel out to straight rolls. This is only if you're both in the darkness, however. If your target is not heavily obscured, and you are – such as the ranger in the bush and the merchant, then the obscured creature will have advantage to attack the unobscured creature, while the unobscured creature will be blind against the obscured creature. This is straight up just how light, darkness, lightly obscured and heavily obscured work in this rule set, printed in black and white (or black and tan, if you're looking at the phb).
You're right in that the spell does not say that the caster would not be affected; the caster absolutely is affected! There's no reason to suppose they wouldn't be, and if they were excluded the spell would certainly say so. There's also no reason to presume any of the things that people commonly presume, which the spell does not say; it's not opaque, it doesn't inflict blinded, and aside from creatures with darkvision not being able to treat the darkness as dim light, and it not being able to be illuminated (that is, turned from darkness into dim light or bright light) by regular light sources or lower level magic, it is just regular, normal darkness, and thereby follows all the rules of regular normal darkness, except where explicitly called out otherwise. Specific beats general. Spells do what they say and don't do things they don't say. Being in regular, normal darkness does not impair you in any way from attacking a creature that is not in darkness... or else sneak attacks from the shadows would not be a thing.
As I said, this is the last post I will make to this discussion; people are free to make their own house rulings or interpretations of things as they like, and are free, for whatever reason, to marry themselves to interpretations of a spell that heavily nerf it, and then bemoan its limited utility, if they want to. As long as players and DMs agree on the interpretation or ruling they're using, and everyone is having fun, that's all that matters in the end.
I'm well aware that the vast majority of players and DMs rule this incorrectly. I'm not expecting to change anyone's mind, and I'm not strongly interested in 'being right' – so much as I'm interested in having a clear and accurate understanding myself (I'm confident that I do, and nothing suggested by others has shown me any way in which I'm in error; if anyone does have something to show that does so, I most certainly do want to see it, and improve my own grasp of things), and then helping to share that understanding to others. The ultimate truth is that this is one of those cases where 'majority rule' is likely to win out regardless of the facts. The same has happened for a number of other spells, especially in adventure league rules – spells interpreted or run incorrectly by the vast majority, and then banned from use as a result. It happens.
I have done my best to illustrate what is accurate and correct here; how anyone else takes that is up to them. I cannot realistically do any more than this, and so I won't comment back to this discussion branch any further (not in thread anyway, I try to be good about PMs). I'm hoping that this is implemented correctly in the game; I don't have much confidence that it will be, because of the majority belief, but I'm hoping that by pointing to the relevant details and rules, it might be. Peace; I've no ill will or aggressive intention, and I don't want to come across as argumentative.
