I can't imagine jet black looking decent in-game. As it is, I tried to make a darker drow with the more grayish undertone and it seemed like the character's skin turned into oily plastic. I played a fair amount into the game before I just couldn't take it anymore; in cutscenes, it looked too shiny, but also flat and doll-like. It's similar with some of the more outlandish colors (the really bright reds, for example). You lose any of the nuance of the skin textures. I imagine it's some kind of shading limitation or I don't know. Call me Lore-Killer, because I left feedback, re-rolled my drow with Ash 4 or 5 instead of 7, and moved on looking fabulous.
I say that like it was quick, but I admit I actually tried out nearly all of the drow tones to see how they appeared in cutscenes. Someone at Larian was probably very concerned for how often I was back in character creator, and I can now get through tutorial mode in a friggin' blink. No one will ask me about the differences between Dusk and Ash, but I know. I know.
The more purpley ones, Wisteria/Amethyst, definitely look much better than the gray tones at skintone 7+ so, yeah, I find it unlikely that jet black would somehow be better than the grays. It'd be flat and fake looking. Sorry to kill The Dream.
Probably a shading/lightning limitation. But this is 2020 we are talking about.