1. Black Americans were taken away from their native specific cultures, whether it be Bantu, Hutu, Berber, etc, and unable to propagate the specifics of their cultures within communities. After emancipation, the only extant culture available was "Black".

2. White Americans generally settled into self-segregating Italian, Irish, German, etc communities or regions, and the original idiosyncrasies of these cultures persisted beyond relocation.

This makes a lot of sense, and I think up until the mid-late 20th century, this was fair, but I do think that sense is losing its strength as American society becomes increasingly atomized.

In 2021, it's equally valid to say that "Detroit Blacks" and "Memphis Blacks" have their own (sure, nascent, but real) disparate cultures, at the same time that it's valid to say that American "Swedish Whites" and "Italian Whites" are far more culturally similar than they were a couple generations ago.

I don't know, we'll see where history leads us, I guess. But the change is definitely visible.