According to the website Wikipedia, the earliest known reference to “Black Friday” to refer to the day after Thanksgiving was made in a 1966 publication on the day’s significance in Philadelphia:

“JANUARY 1966 -- “Black Friday” is the name which the Philadelphia Police Department has given to the Friday following Thanksgiving Day. It is not a term of endearment to them. "Black Friday" officially opens the Christmas shopping season in center city, and it usually brings massive traffic jams and over-crowded sidewalks as the downtown stores are mobbed from opening to closing.”

The term began to get wider exposure several years later. See:?  http://en.wikipedia.org/wiki/Black_Friday_%28shopping%29