To all of the racist out there....
The earliest uses of "Black Friday" come from or reference Philadelphia and refer to the heavy traffic on that day, an implicit comparison to the extremely stressful and chaotic experience of Black Tuesday (the 1929 stock-market crash). The earliest known reference to "Black Friday" (in this sense), found by Bonnie Taylor-Blake of the American Dialect Society, refers to Black Friday 1965 and makes the Philadelphia origin explicit:
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 Black Friday began to get wider exposure around 1975, as shown by two newspaper articles from November 29, 1975, both datelined Philadelphia. The first reference is in an article entitled "Army vs. Navy: A Dimming Splendor," in The New York Times:
Philadelphia police and bus drivers call it "Black Friday" - that day each year between Thanksgiving Day and the Army-Navy game. It is the busiest shopping and traffic day of the year in the Bicentennial City as the Christmas list is checked off and the Eastern college football season nears conclusion.
The derivation is also clear in an Associated Press article entitled "Folks on Buying Spree Despite Down Economy," which ran in the Titusville Herald on the same day:
Store aisles were jammed. Escalators were nonstop people. It was the first day of the Christmas shopping season and despite the economy, folks here went on a buying spree. ... "That's why the bus drivers and cab drivers call today 'Black Friday,'" a sales manager at Gimbels said as she watched a traffic cop trying to control a crowd of jaywalkers. "They think in terms of headaches it gives them."