How much of the Black God (Kurokami) manga was adapted into the anime?
This is probably not the answer you are looking for. But having seen the first episode of the series, I can say that the divergence begins in Chapter 1/episode 1 as Keita's character is completely changed. In the manga he programs video games while in the anime he is a high-schooler. In the manga he is an arrogant prick; no such attributes to the anime character.

Since the anime was produced when the manga was still being written, the anime had to completely diverge from the manga's plot at one point. This, I'm told, happened in episode 12. I'm not entirely sure which chapter or volume this represents. The ending is completely different from the manga.