It be like yall dont interact with ppl outside the internet. AA can be used as a blanket term for all Americans of African descent (it has historically been used this way) or AAs that can tie their genetic roots back Africans brought to American from the slave trade. "Black" is the race all ppl of African descent fall under but there are varying degrees of "Black" as there are with any race. We only mention it in American, ie Black American, because this is a primary white nation built by white ppl for white ppl. You wouldn't go to Nigeria can call ppl Black, its redundant.
This person your describing is an African American with, for an example, Nigerian roots or a 2nd generation Nigerian - American. No matter which, this person is Black.