from wiki:
Origin of the name "Black Friday"

Stress from large crowds
The earliest uses of "Black Friday" 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) or other black days. The earliest known references to "Black Friday" (in this sense) are from two newspaper articles from November 29, 1975, that explicitly refer to the day's hectic nature and heavy traffic. 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 made even more explicit 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."

Both articles have a Philadelphia dateline, suggesting the term may have originated in that area.

=====

The first time I heard the term "Black Friday" was the day after Thanksgiving 1974. I had to drive out to somewhere in West Philly to some 'special' plumbing supply store--I used to help Dad do plumbing at home--to hopefully find some odd part, and they had it. It was raining that day and I told the man behind the counter that traffic was "nuts out there." "Yeah, they call it 'Black Friday'." "Black Friday?" "The day after Thanksgiving. Traffic's real heavy; they say everybodies going shopping." "You mean like shopping for plumbing supplies?" Laughs and good-bye.

I figured out that it was 1974 because I remember hearing a solo George Harrison song on the car radio and it went exactly with the windshield wippers. Then the DJ said Harrison's new solo album was coming out the end of December. According to wiki, Dark Horse was released December 20, 1974, and I got the album that Christmas.