re: #3 HappyWarrior

Exactly, in Plessy Vs Ferguson, Homer Plessy was arrested for being on a whites only railroad car. It was as you say law.

And the mindless tropism response of a devout libertarian that weakening the state governments would have alleviated Jim Crow also fails.

With sufficiently weak state governments, oppression of blacks would have been enforced by local custom (e.g., lawless gangs of KKK thugs). This might have been even worse than Jim Crow.