The term "Black Friday" originated in Philly in the 50s and referred to how much it sucked to work retail on that day
The "it's the day retailers are in the black" thing is pure malarkey.
"For years." But how many years? Ben Zimmer collects some evidence that the term was already in common use by 1961 (common enough that Philly merchants were trying to change the term to "Big Friday"), and passes along an interview with Joseph Barrett, who recounted his role in popularizing the expression when he worked as a reporter in Philadelphia:
In 1959, the old Evening Bulletin assigned me to police administration, working out of City Hall. Nathan Kleger was the police reporter who covered Center City for the Bulletin. In the early 1960s, Kleger and I put together a front-page story for Thanksgiving and we appropriated the police term "Black Friday" to describe the terrible traffic conditions. Center City merchants complained loudly to Police Commissioner Albert N. Brown that drawing attention to traffic deterred customers from coming downtown. I was worried that maybe Kleger and I had made a mistake in using such a term, so I went to Chief Inspector Albert Trimmer to get him to verify it.