Again its a matter of record they weren't treated well in the north either, yet it didn't stop them when offered individual freedom. Neither did the fact the north had slaves.I would think that the fact that freed blacks were not treated well in the South would argue against voluntarily going the Confederate Army. Southern blacks weren't familiar with Northern racism but they certainly had a score to settle with Southern whites.
A chance at individual freedom whether for a slave holding nation that still wont treat you as an equal, is still an opportunity to better oneself from slavery, so they took it, whether for the US or against the US with GB.......as they more then likewise would have also for the CSA if it had been offered. To them it wasn't some abstract political issue for others, but a chance to better oneself