How about the *ultimate* emigration? No, I don't mean outer space, that would be clearly ASB. But mass suicide might not be....
ITTL, an African American demagogue, possibly a religious leader, starts the "Hell is better than America" movement, encouraging mass suicide as an improvement over life in the super-racist United States. Although I could put an alt-MLK or an alt-Malcolm Little in this position, I think the timing would be wrong. 1920 or so is what I have in mind.
So, anyway, this demagogue kicks off the "Leaving for Hell" movement by orchestrating the simultaneous suicide of 1,000 African Americans, including himself, at a New Year's Eve "Nightwatch" event (common in AA churches at the time). The event is geographically distributed among many locations so as to be harder for outsiders to prevent.
The more extreme racists in the White community respond with, "Good riddance!" The moderates say nothing. A few more intellectual Whites bemoan the tragedy but no one cares what they say.
In the wake of the leader's "Departure", other seemingly spontaneous "Departures" ranging from dozens to hundreds of African Americans each happen every day for the next few years, until the number of African Americans left is so low the White racists no longer really care about their presence anymore.