You are correct. If you have Disadvantage from lack of vision, you still have Advantage if you melee a prone target, which gives a normal roll.
In BG3 currently, Disadv. and Adv. do not cancel out if there is more of one than the other. So if you have 2 Disadv., and 1 Adv., the game gives you Disadv. In 5e, they do not stack and just cancel each other no matter (a debatable rule in my opinion, but officially it is the rule).
nb : in current BG3 and 5e, if the creature is unconscious/KO, than it is an automatic Critical (a DM would probably disagree if you have Disadvantage at the moment).
Last edited by Baraz; 27/10/20 03:45 AM.