If and when slavery becomes massively uneconomic it may die off, certainly not before well in to the 20th century. Having said that given many more decades of slavery and attendant color based racism, if and when slavery goes away the position of the former slaves is going to be terrible. Apartheid etc would be the least of it - once they are no longer slaves/property, food and shelter will become problematic. This is not saying slavery is good, but given the capital value of slaves most owners would neither have them sleep in the open nor starve them. A CSA with a large "free" black population is going to be having tremendous internal strains. A slave may want to be free, may be thinking of rebellion but compared to someone who is starving and seeing their family starve their drive for immediate and violent action is much reduced.