What the U.S. calls "races" would simply be considered subcultures in the rest of the world. (Bordering on ethnicities, but not really - they're way too integrated within mainstream culture to be true ethnic subdivisions.) You can acknowledge subcultures without clinging to the absurd notion (that is, absurd to much of the civilized world) that "race" is a legitimate term at all, even for a socially and culturally-bound construct.
I don’t think that’s correct considering people from Nigeria, France, or N-th generation Americans can all be considered “black” provided they have a certain set of physical traits even though these people very likely have very different cultures. So the American notion of “race” spans cultures and it is derived from physical traits, not cultural artifacts.
Whether African immigrants qualify as "truly" Black is actually a very contentious point within Black culture itself, and that's despite widespread solidarity with Africa and Pan-African ideals. This makes 100% sense if you regard this U.S. notion of "race" as a pure social construct, something not dependent on any fixed set of physical traits or purported ancestry.
Within American culture, it’s not contentious that dark-skinned African people are “black”, certainly no less so when they immigrate to the US. But that’s race.
There is also a distinct notion of “black culture” which is a subset of American culture (people who identify with black culture also tend to be racially black, but not every racially black American identifies with black culture). That there is a “black culture” doesn’t mean that the American notion of race is incorrect.
