Interesting idea, though it has been brought up many times.
I've been thinking of an alternative take on this.
There is an old saying from somewhere that to end the resistance by plains Indians to American westbound immigration, one had to kill every last buffalo, the idea being that removing their food source would cripple their culture, which it did.
How about a different take?
How about to remove the last bit of southern resistance to American rule, one had to remove every last Negro from the south?
For example, many freed blacks were stuck as sharecroppers, doing the same job as before and largely continuing previous social patterns.
How about a surviving Lincoln issues the old "forty acres and a mule" offer with the take that the freed slaves move west into the still-largely empty plains states/territories of the Dakotas, Wyoming, Montana, Colorado, New Mexico, Idaho, etc, etc, etc.
Lacking their age-old labor force, the "States-rights" aristocratic champions of the south would lose their income source and a shift in power to the middle-class and lower classes, permanently destroying the previous social fabric?
The blacks become a propertied class and the southern gentry is destroyed.
Kill two birds with one stone.