I've been wondering about the same a while ago actually. I really hope it's not an offensive thing to ask, I just want to make sure to do it correctly
I never got around to making a thread about this, but I did make some examples of how portraying dark skin in only black and white could look like.
I'm definitely not a fan of A, lol - first off, it's not very readable, secondly, all the extra work with changing the lines' color it requires... Thirdly, it just doesn't look particularly good to me (also looks more like some fantasy pitch-black skin, rather than natural human black skin) and I don't recall seeing any comics/manga that did it this way, too.
B on the other hand might look like white-washing the character.
Of course it's still going to be clear that the character is dark-skinned in chapter covers, bonus art, etc., but I don't want to leave a wrong impression.
C breaks the monochrome palette, and requires bringing in a midtone color in addition to b&w. Which is fine (and is imo the best looking example)... but not if you're set on a style that uses only black and white, so it's a work-around, rather than a solution.
D is... technically still using only black and white, but I'm honestly not a fan of using screentones in digital art (might be just personal taste, honestly), and it's also not as readable. Though the latter might be because the screentone I used isn't a particularly great example.
Keep in mind, all of these are my personal opinions, and I can't tell anyone which way is right or wrong.