aka brown sock. When a person is extremely promiscuous when it comes to anal sex they have higher odds of getting a pink sock, therefore when their bowels are so often exposed to the elements they become blackened from exposure, dragging through the dirt, etc.
That slut takes it up the ass so often you have to watch out you don't step one her (as a gay man would refer to another) black sock dragging in the dirt.
In jail, you get many varieties of prolapsed rectums, and the black sock occurs when one penetrates or sodomizes a north american land ape, a.k.a. an african american.
"damn dawg, that bitch be so fine lookin, he be asking fo da black sock tonite"