A much more racist US taking ideas from South Africa might decide to create a "Bantustan" in the most undesirable/resource-poor regions of the south or west.
Alternatively, a Black Autonomous Socialist Republic is mentioned as having been established in Jello_Biafra's Reds!
The main issue is that African Americans simply do not have the numbers or power to unilaterally secede against the US Government. Therefore, barring a collapse of central government, the black state will have to exist at the mercy of the white majority. (And even if there was a collapse in governmental power, they would probably not be able to form a coherent state unless they already had a unified organization and/or outside help)
Agreed.
I recall a reference in some ancient sci fi book to some "United Ghetto" Nation of blacks, perhaps after a Marxist/Race uprising...
I remember thinking it was a dumb idea, but PERHAPS, a far more racist America, (one where the Klan suppression did not work?) might set up a Native Indian Reservation style system with Black majority areas set up as "Bantustans" as a form of extreme segregation.
Perhaps in response to international condemnation for Human Rights Violations from a more progressive rest of the World?
Which would likely include as you suggest the poorest, least desirable regions of the South...
POD no later than the mid 1920s I would think.
Build from the height of Klan Power so that it becomes instead of a part of history, a continuing part of the Establishment.
Very unpleasant timeline, no doubt about it.