this comes from the ancient folk lore about
the black man who would have
sexual intercourse with his mother after a long day of hunting. he would always finish the load in his mothers bum when the sentence "black fuck incest would
cum in my ass" uttered from his mothers lips. you would normally use it to describe a black man coming home with a food stamp who is so horny that he would fuck his own mother.
White man: "Hi buddy, let's go collect the monthly
food stamps!"
Black man: "Cuuhhh, I already got it cuuuuhh. This is a good time for black fuck incest would
cum in my ass and marijuana!"