They didn't have much of a choice.
People without a penny to their name managed to migrate from across the sea to America. Ditto for people in the Eastern part of America, who moved West. Why couldn't blacks mass migrate to the North? To this day, the descendants of the freed slaves still live mostly in the South. Either they weren't treated that badly down here, or they knew they'd be treated exactly the same way by New Yorkers and Vermonters and, yes, Kansans.
There was no significant movement of blacks to the North until after WWI, when industrial jobs attracted poor blacks and whites both to some Northern areas. And what was one of the first things blacks did when they arrived in their new locale? It was to charge the local whites with "racism".
Ever hear of Brown vs. The Board of Education of Topeka, KANSAS?
In Detroit (which you once noted was part of the United States, though we might wish it wasn't), black kids grow up being told that the city was run by racists until blacks took the place over during the years 1968-1972, when the white population largely fled to escape black violence.