Thing is though, "polish" and "irish" correspond to, clearly, the country's history only. "Black" refers to literally anything about black people, which is a totally different idea.
You seem not to be aware that these things exist. They very much do. African-American museums celebrate the community of Americans descended, primarily, from people brought here as slaves from Africa. They have an immense shared culture and heritage, stemming in large part from the way the law of this country segregated them until just a few decades ago, and thus have a cultural identity here more coherent than those of (say) Irish-Americans (who do indeed have a museum, and most of whom couldn't name 3 cities in Ireland or tell you what Gaelic is).