Possibly better integrated blacks? Even in a scenario where the South secedes successfully, there's going to be slaves escaping over the border to the North. They may provide cheap industrial labor that was otherwise given by immigrants, and so create the effect of the Great Migration of rural blacks to industrial centers in the north a century ahead of schedule.
Of course, the sudden influx of blacks into the North's industrial center may not result in their integration, but could be what drives the US to end it's anti-immigrant isolation so as to keep 'those people' out of the northern cities![]()