The "Old Black Man", also known simply as "Old Man" is an acquaintance of the gang.
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 share a bed with an old man for a year. When Frank produces the man in question, 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 Old Black Man under the Bridge. It's also revealed that his real name is Carl.
Old Black Man originally lived with his wife, Ruth, in the nursing home where Scott Bakula was employed. At an unknown point in time, he went missing and became lost on the streets of Philadelphia (presumably due to age-related dementia). Old Black Man eventually took up living under the Bridge as a homeless man. Ruth and Old Black Man have two children together, whose bodies are occupied by Dennis and Dee during "The Gang Turns Black". After reuniting with Ruth, Old Black Man returns to life in the nursing home.
Old Black Man is known to spend an abnormal amount of time sleeping. During the period when he spent his nights in bed with Mac, Dennis, and Dee, he would return to the Bridge in the daytime to sleep on a mattress.