You could also have a tech that allows black union troops for the north. Emancipation could be a tech or something for union which maybe gives them a certain amount of free black divisions are something like that. That could be a really interesting advantage and part in the game.
Just so you know, there actually were black soldiers in the Confederacy. Sure there were not a whole lot, but they were there. Oh, and sad to say it, but most of the black men in both armies were used as laborers only.