Many black Americans today likely had relatives who were slave owners through interracial marriage, or (gasp) ancestors in Africa who owned slaves. It's not like it was a dinstincly American practice.
America, apparently, was the only place on earth that ever had slaves, in all of history. It started with us and is all our fault. Only black people have ever been slaves. No white person was ever a slave. Only white people ever owned slaves. No black person ever owned a slave. Only white people ever took or sold people into slavery. No black person would ever do that...........
Many blacks were slave owners. Interracial marriage wasn't needed.
Some freepers who know about history in the south could point you to that information.