If you're gonna soak beans before cooking them then you need to soak the black beans for 8 to 10 hours before you cook them.
Then remove the old water and replace the old water with new water before cooking the beans and they should take less time to cook and also be less gassy than before.
The best thing to do is to just put the beans in a slow cooker or other pot and then let them soak overnight while you're asleep and then they will be ready to cook in the morning.
Black beans and other beans can take from 6 to 8 hours to fully cook and soaking the beans can help them cook in less time.
I always just cook my beans in my slow cooker while I'm away at work and then they're ready to eat when I get home and ready to eat supper.