Here is a cite related to the term “Negro” being exchaned for “Black.”
I found it hard to find other cites, but my understanding (which other posters have put forth too) is that “colored” was the polite, unoffensive term until the 1940s or so, at which point “Negro” was introduced. This term, in turn, started to fall out of popularity at the end of the 1950s (as per the cite), and the term “Black” was adopted. Note that the terms “Negro” and “Black” were endorsed by black people themselves (at least the latter term, for certain). Then, in the early 90s, “African-American” was introduced.
Personally, I can’t see why “Black” wasn’t good enough. Most black people I have known seem satisfied with it. It’s simple, and everone understands what it means.
Re the OP, I don’t think we need any more terms to divide us up into categories: black and having slaves as ancestors, black and having sub-Saharan Africans as ancestors. No thanks.
First, the color issue is meaningless. The race thing. Especially in America, we need to rise above this. And we are, slowly but surely. The fact that someone is black but from Africa or not isn’t all that relevent.
The cultural issue IS more relevent, but this is not connected to race directly. Sure, there’s a high correlation there, but it’s not definite. Anything you can point to as “black culture” in the US is related to “white culture” and European/Asian/African culture; and anything you think of as “white” is still going to have that Southern/Black/African influence. “Black culture” is open to whites (Eminem) and “white culture” is open to Blacks (Charley Pride, and country music has a huge black influence anyway). And for that matter American culture is open to the whole world.
The great thing about America is that anyone can join. Africans coming now certainly do not need to be labeled “a different kind of black people.” Really, it’s time to rise above this primitive way of thinking.