When an American says "black", they often mean "African American". The reasons why one would use one word over the other is usually more about social conventions between synonyms and less about trying to describe different ideas.

It's really self-centered when we use "black" to mean "African American". It's strange when it goes the other way. You'll hear some Americans call Idris Elba an African-American.

When I say "Black", I mean "Black". If I want to single out Americans, I say "Black Americans."

I'm all for dropping the term African American, unless it's referring to a recent immigrant. It's a misnomer from the past, that for some reason people consider PC.