'history shows that crushing slavery in the American South, the degree to which it did not lead simply to the strengthening of the American workers' movement and overcoming capitalism in the U.S., was a highly ambivalent phenomenon. In many ways Jim Crow racism combined with intensive capitalization in the South was indeed "worse than slavery" '
I wonder if you aren't rather telescoping the historical events here.
Jim Crow was not the immediate consequence of the Union victory, but itself the outcome of a social struggle that followed after (about twenty years after). Immediately after the war there was reconstruction, and more than that there was radical reconstruction, with a genuine attempt on the part of the occupying Union forces to democratise southern society.
So for example there were scores of African-Americans elected to public office in the south between 1868 and 1877 http://en.wikipedia.org/wiki/African_Americans_in_the_United_States_Congress#In_Reconstruction_era_2
The Jim Crow laws are usually dated from 1876, just before the 1877 compromise under which Union troops were withdrawn from the South. In 1875 a Civil Rights act outlawed discrimination in public services, which was overturned in 1896.
Marx was surely sincere when he raised the cause of the north in the American civil war. It was not some kind of tactical jiggery-pokery, but an understanding that in all senses reaction was on the side of the South, and progress on the side of the north. The labour movement that wavered on that question would not be worth the name.
It would be wrong to say that the Jim Crow laws in any way qualified the advance that the northern victory in the Civil War represented, any more than Stalin's reaction qualified the 1917 revolution. Jim Crow was the reaction against the advances made in the first years of the reconstruction, and the latter is not reducible to the former.