The euphemism treadmill is basically arbitrary most of the time. For example, "people of color" is very PC right now, but "colored people" is considered KKK-language. It is what it is.
Also black people is a kind of strange term. Pretty much all black people are okay with it, but a lot of white people are weirdly afraid of saying it, especially in formal settings.
Black is a useful term for referring to people of African descent who aren't African-American, e.g. Caribbean-Americans.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.