I read the book back in 2008 or so and it was pretty good. I had no issues at the time with God as a motherly black woman. The book wasn't really about who the characters were portrayed as; it was more about what messages they were trying to get into the main character's head. I think I still have a copy laying around somewhere so I might reread it.
God is Love and thus his physical appearance is not relevant. It is absolutely fine for God to be cast as a black woman and I say this as a Catholic theologian.