I could've sworn that one of the appeals of the Mexican Far North was the fact that nobody knew your racial background and thus you could go there and see what you make of yourself there. And at times, outright forgery was involved in your ability to claim yourself as white.
It's definitely erroneous to believe that Mexico was a land of racial freedom and colourblindness. And there's certainly no way a sudden influx of black slaves from the US will be treated positively more than some individual arrivals. And with freed blacks, what might make them move to Mexico (a certain part of it at that), when they already had rights (which granted were gradually eroded from the 1830s onwards) in the US? There's always Caribbean freedmen, but Mexico will be competing with Liberia and Sierra Leone for both them and the US freedmen.
It's definitely erroneous to believe that Mexico was a land of racial freedom and colourblindness. And there's certainly no way a sudden influx of black slaves from the US will be treated positively more than some individual arrivals. And with freed blacks, what might make them move to Mexico (a certain part of it at that), when they already had rights (which granted were gradually eroded from the 1830s onwards) in the US? There's always Caribbean freedmen, but Mexico will be competing with Liberia and Sierra Leone for both them and the US freedmen.