I have always wondered the same thing. I have read and been told that it gave/gives them a sense of community or control over their lives that that they were missing. I went to a Boston athiest meeting just to meet and speak with a few athiest blacks. One 50 something woman(a believer) assured me that god existed and that she has the personal experience of god. How do you argue with that?

I'm given to understand that slave owners permitted religion because it kept the slaves servile and made them easily controlled. Whether or not that was a conscious strategy, I suspect it was in fact the case. I would imagine that for some the notion of slaves singing as they worked even allowed them to suppose that the slaves were happy.