I'm trying to remember the name but I was reading a review for a book a few weeks back that was arguing that unofficial slavery was still in operation up until the Great War in some states in the south. Basically the police would arrest blacks for fairly minor crimes such as vagrancy, take them before friendly judges who would convict and sentence them to hard labour, and they would then be farmed out to local interests like mines or forrestry businesses where they were effectively treated as slaves. The author argued that it went on for decades after the American civil war until mechanisation made it uneconomical.
Depends on if the ruling classes thought they could still gain a financial benefit from the pool of labour they represented. If yes then I could see them introducing legal barriers to people emigrating to Mexico or the US, if not then they could encourage - either officially or just unofficially - blacks to leave to get rid of the useless mouths that need feeding.What would happen to the slaves then? Immigrate?