In my experience, African American Catholics are rare in the United States (and by African American, I mean those who are of African descent, but who can trace their ancestry a few generations as born and bread in America, perhaps up to colonial times).
However, African Catholics tends to be much more common. I find that if I see another black person in a Catholic Church, that person is usually either from Africa, or was born to parents who immigrated from Africa (I, for instance, was born to Nigerian parents).
They also tend to have their own parishes and services which celebrate the Mass in their own language (and their own style, which for us, includes a bit more “moving to the left and to the right”, as Cardinal Arinze eloquently put it. in one of his interviews). Their culture is different from African Americans, and like other immigrant cultures, they tend to keep to themselves.