They don't actually follow up on it, which I find a waste and annoying, but that blackness points to something disrupting her soul's proper journey, which is I'm guessing how she was able to be fully resurrected. The way Kethric talks it's clear that Myrkul directly acted to bring her back, and I'd say that even if he's not a full god, he's still a death entity with power that goes beyond even a level 20 cleric, so I don't think him being able to break or bend some rules to fully resurrect Isobel is unbelievable.
It�s unbelievable, at least for me as a person somewhat knowledgeable about the lore. Yeah it�s a magic fantasy world, but it has rules even gods can�t bend. Resurrection 100 years later is impossible, there�s no body, the soul departed�finita. Reincarnation, as in taking a petitioners soul and implanting it into a newborn with all the knowledge from previous life - yes. But the soul would most likely be VERY unhappy having to live a mortal life again, after experiencing afterlife.
That aside - the whole thing is just convoluted, over complicated and unnecessary. Why resurrect the dead, instead of just having normal villains? The villainous trio, along with the emperor and the crown nonsense is just a fail.
I do like Orin though. A proper psycho maniac. She is the best of the trio. Just bcos she�s pure, without too much unnecessary over complications (altho larian did make her a child born of incest rape� why? Can�t keep things simple, eh?).