Um... why would blacks be more left-wing in a timeline where they aren't subjects to state governments/cultures which actively sought to stifle their economic improvement? If anything, blacks as a group would be LESS left-wing, since they would on average be wealthier due to less generational poverty and lack as much of a need for identity politics. Also, if blacks as a whole voted as often as whites during the later decades of the 1800's and throughout the 1900's, you can bet the Southern Democrats are going to shift policies to try to snag up some of their votes.
Though, I imagine in this timeline you might see some "White Flight" of poor whites from the south to the Mid-west, Prairie States, and North, creating a more sharp racial regional divide (as there'd also be less motivation for blacks to migrate north).