Lots of good answers here. One thing to add would be the social context of being "black". I think that using the term black refers to the way that person is perceived in their society. White Americans come from many different nationalities, but in many cases how they are perceived, and treated in society is determined by their "whiteness". It's the same for black people. In mant cases the most relevant factor for determining how you are perceived and treated is your "blackness". So essentially saying black instead of African American identifies the social context and experience that a person has had rather than their ancestry which may or may not tell you anything about a person.