Slavery was legal in all of the United States for centuries. So narrowing down the category to descendants of southern states actually excludes many people whose ancestors were enslaved outside of the South. I suggest 'descendants of slaves' instead.
And now back to discussion. Sorry, my inner historian had to be heard!
And descendants of people enslaved outside of the US - the Caribbean, Brazil, other parts of Latin America - have a different cultural background than those whose ancestors were enslaved here.
I have heard a few African immigrant acquaintances say they were ambiguous about the term 'African American'. They are indeed African, and are now American. However African American refers to the collective experience and history of the descendants of people from Africa who were brought to the US centuries ago and enslaved. It is not a good 'fit' for people who recently immigrated from Africa to the US - their ancestors were not slaves and their historical background and cultures are very different from how African American is typically understood. Of course, this is a tiny, anecdotal sample and can't be assumed to be representative.
But, yeah, ultimately it boils down to 'politely ask preferences and then use said preferences'.