The word appears 4 times.
The novel takes place in the time of the south were Faulkner spent his youth. The word is used to describe the skin after an accident where someone got burned. Their skin turned black, and they use the word to describe it.
I thought she had a quarter or a dollar at the most, and that after she stood around a while she would maybe buy a cheap comb or a bottle of nigger toilet water, so I never disturbed her for a minute or so except to notice that she was pretty in a kind of sullen, awkward way, and that she looked a sight better in her gingham dress and her own complexion than she would after she bought whatever she would finally decide on.
"Do you want some more water on it, Cash?" Dewey Dell said.
Cash's leg and foot turned black. We held the lamp and looked at Cash's foo and leg where it was black.
"Your foot looks like a nigger's foot, Cash," I said.
"I reckon we'll have to bust it off," pa said.
.... While treating a burn Jewel was lying on his face. His back was red. Dewey Dell put the medicine on it. The medicine was made out of butter and soot, to draw out the fire. Then his back was black.
"Does it hurt, Jewel?" I said. "Your back looks like a nigger's, Jewel," I said. Cash's foot and leg looked like a nigger's. Then they broke it off. Cash's leg bled.