Can you ask a black American what country of Africa they originated from or would that be considered insulting/offensive?

You would ask a Black American what country of Africa they came from? You are a Republican and voted for Trump, didn't you?

Slavery made knowing that for certain impossible. You may recall they weren’t allowed to keep family names or speak their native languages. DNA tests can indicate the nationalities involved, but not the actual ancestral country of origin.

It depends on a few factors.

I can see how that can be problematic

They're probably so integrated by now that it's many countries. Some European in them too. Don't ask them. It's rude and pointless.

It’s offensive.

Is the Black American your friend? or just some stranger on the street? It matters.

Both insulting & offensive. Remember, the boundaries and walls that define countries are false. They are created by people who have no right to claim what they claim. We all originate from the same place and have no choice in exactly where. So, to claim one is more superior to another Is just crap.

Probably not but as a white American I’m not sure of all background my ancestors hail from I imagine that’s most people.