What about an ATL branched off from TL-191 (or a similar TL) in which the CSA's Red Rebellion is succesful and forms a new Communist government. But then a black leader like the character Cherry (hey a communist female leader could happen) who is VERY racist against white people takes power, instigates a Stalinistic regime, and commits genocide against the countries white anglo-saxon inhabitants.
I think the creation a of a "Jazz & Genocide" culture could make some pretty cool bad guys. Plus the concept of a government controlled by black people who mistreat white people is a nice subversion of the trope if you know what I mean. Kind of like Marvel comics House of M series in which mutants rule the world and humans are an abused minority.
...not so much "mistreat" as much as go Khmer Rouge on them, if I'm interpreting this correctly?
One tragic course could be a campaign against fair-skinned black people, when in fact they themselves had been treated as black by white people all along, of course.