What if.....? Part 2 The North
The social implications to a separate peace would have been in a word chaotic. Whereas the Abolitionist movement was a real political power in certain areas, it was in no way universal. In fact, many of the abolitionists were not enamored with the idea of living shoulder to shoulder with blacks. Lincoln himself was actually interested in the idea of returning freed slaves to Africa.
http://en.wikipedia.org/wiki/History_of_Liberia Working class whites, especially immigrants, in the north were certainly opposed to large numbers of freedmen showing up and competing for work. In the aftermath of a “lost” war this feeling would have intensified. The ensuing social unrest in urban areas, and its fallout, would have long lasting negative effects. It is highly unlikely that the northern black population would have taken to this kindly, especially considering many thousands of them had taken up arms to free their families still in bondage. The resentment on both sides would have been considerable and surely would have resulted in multiple acts of brutality. The US government would have had to step into this chaos, and it is very unlikely that they would have favored the black population over the white in their interventions. Victorian age thinking did not see the blacks as any way equal to whites, and more than likely the Back To Africa movement would have picked up traction and become official policy.
Another question arises about the disposition of the thousands of runaway slaves the US Army had under its control in occupied territories throughout the south. While some US commanders were confirmed Abolitionists most were basically ambivalent about the black man. During the war runaways were welcomed by the US Army because it deprived the enemy of labor and at the same time gave the Army a thankful and motivated workforce which was happily exploited throughout the conflict. What would have happened to these former slaves as the Army pulled out? Chances are very good that they would have been left to their own devices with tragic results as Confederate troops reestablished control.
I believe that the northern black population as a whole would have felt great pressure to either go to Africa or perhaps migrate west. The cost in lives would have been high either way they went because the black population at the time would not have generally had the resources they would have needed to ensure success.