It's a hint that we subconsciously pick up. Because capital letters indicate a group with a name and an identity ("American", "Bavarian"), which strengthens the idea that being black is an identity. There are many more, but if you try to talk about these hints, you will just sound like a crazy person. Any tech-savvy person could probably figure out a way to search the articles of the last 20 years and see how often there are articles in which the words "Black" and "white" appear, in contrast to "black" and "white".