It’s an interesting grammatical thing. In English, proper nouns are generally capitalized. Where proper nouns are names of specific things, not generalizable ideas. Like Bob, England, The Tribune, Christianity etc are proper nouns, while cat or guitar or car are not. This is extended to proper adjectives, which are generally derived from proper nouns but not always. So like “the man was English”. We capitalize English because it isn’t just a descriptor of a trait, like fat or green, but because it is describing membership to a nation, and nations are proper nouns. Blackness describes a nation type relationship, and when you say someone is Black, you are not saying that the are literally the color black, but rather belong to a Black identity or nationality. In the same sense that you say someone is Jewish or Protestant or Welsh, not jewish or protestant or welsh. Idk English is weird.