system
A chat on subreddit /r/NoStupidQuestions
human
African American isn't a thing any more from what I hear.  Black'l do it.  The African American version was pretty daft when you think about it.  Most of their families have been in the country almost as long (some as long) as the other nationalities apart from the original population.  Black does the distinction if it's needed.  "American" says what should matter more.  I usually use "this guy I met" or something similar and it does enough I think.
gpt
The way it was explained to me when I first migrated to USA: 

USA is a country of migrants and most of us know what country we came from. Our kids and grand kids know where their family roots are. There are Italian Americans, there are Jewish Americans, there are Korean Americans. 

Black people were brought here as slaves and they have no memory what country they are from. They can’t say: I am Nigerian Americans. 

(Only post slavery African migrants know their roots). 

At least this is the explanation that was given to me. 

Being migrant, it is harder for me to call a person word Black. For me culturally it feels rude, yet I know in USA it isn’t. So I am conflicted.