Too much exposition at first (the dreaded data dump), but once you got to the point, your writing doesn't hold back. Brutal and efficient. Good writing, horrible content. I didn't score, but it would have been a five for writing, one for content (non-con not being my thing).
You will get (are getting) pounded score-wise because the violence (rape) is primary, the incest is secondary.
Also, what's being coloured got to do with it? The race aspect, as I read it, was irrelevant.
Or were you deliberately going for the trifecta to be controversial? I hope not, because your writing is strong enough you don't need such cheap tricks.
I don't like noncon so I didn't read much of your story. That said, I read enough to spot what I consider three serious problems:
1. There was no set up for the grandmother physical assaulting her grandson. She had spent a lot of time raising him and had never used physical discipline then, so for her to suddenly beat him into submission was jarring
2. There's very little what of what the narrator was thinking during the physical assault, subsequent forced sex and then the time afterward. His grandmother assaulting him seems to have no more effect on him than if she called him a few choice names
3. There's already so many implausible things in the story that adding "a clit about three inches long and as big around as my thumb" made the story farcical