The "Old Black Man", also known simply as "Old Man" is an acquaintance of the gang. He is portrayed by Wil Garret.
Background
In "Mac & Dennis Move to the Suburbs", Frank wagers that Mac and Dennis, being "city scum", will be unable to live in the suburbs for even a month. If they prove him wrong, he will pay their rent for a year, but if they fail, they will have to sleep in the same bed as an old man for a year. When Frank produces the man, Mac and Dennis are shocked to learn that he is not only old, but also black. Mac and Dennis are just about to successfully win the bet when they realize that doing so would mean an entire year living in the suburbs, and they purposefully lose at the last moment. Due to her taking a side bet with Frank, Dee also has to spend the next year's nights in the same shared bed.
In "The Gang Turns Black", it's revealed that Frank originally found the old black man under a bridge.