Black Cosplayers really aren't all that common. Some black nerds may use cosplaying as a way to get closer to white people, but I definitely wouldn't say that as a majority. But it does happen. It's also a lot easier for you to end up as a token in that community, since it's mostly swarmed with white people. Unless you have a squad already or you just opt out altogether, chances are, you'll end up being that lone black person in a group.
Plus, you always have to deal with the fact that if you're black and cosplaying as somebody who isn't black, you're going to get a lot of shyt for it. At the end of the day though, they should do it out of the love for the characters and the passion for dressing up. Not to win over the acceptance of people who don't fukk with you to begin with.