According to Wikipedia, this name has been used for the day after Thanksgiving since 'at least 1961. More than twenty years later, as the phrase became more widespread, a popular explanation became that this day represented the point in the year when retailers begin to turn a profit, thus going from being "in the red" to being "in the black"'
For the benefit of my UK readers (which is most if not all of them): Thanksgiving is the last Thursday in November.
© Haydn Thompson 2017