It is possible this could have worked once. But now, "Black" is an ethnic group, a people with their own culture, language, customs, and cultural history. As Amiri Baraka wrote "Black is a country." Race is not real as you note. Race is a social construct. There is biologically only one race. But the racial caste system exists and is not close to being taken apart. But what that caste system has made is a people. African Americans. Black people (capital "B."). That is why Black is capitalized. It is the same as Irish or Italian or Polish.