I believe that mass persecution produces a communal dependance on God. When do we really turn to God the most (if we're honest)? When we need something or are hurt with seemingly no way of escape. Slavery did that for black americans. Continuos persecution did that for the Jews etc....
With that said, I dont believe that Black people are actually more religious than others. I just think that the way we "do church" is distinct and our "outspokeness" about God is in line with our collective outspokeness as a group. (I know that this is partially a sterotype but we cannot deny that a large portion of our "race" tend to be more demonstrative, loud, and given to emotional repsonses-good and bad.) So my explanation above is more in referance to maybe why religious activity/belief is expected in our culture moreso than in other cultures.
Did that make any sense? lol