The term "Black Friday" is said to refer to the day that so many people shop in US retail stores, the day after Thanksgiving, that retailer's ledgers go from red (debit) to black (profit). But this appears to be a more recent use of the term which has been in use since the middle of the last century to mean something different. It was still used by retailers and other concerned with the hoardes of people shopping on Thanksgiving weekend, but not in relation to the ledger books. Rather, "Black Friday" was a bad day because it was when all those obnoxious shoppers, brats in tow, came into the downtown shops (in those days there were downtowns with shops) and annoyed everybody. And, the term seems to have been born and used for a long time nearly exclusively in Philadelphia. The term may have even started with the city police. And, the big Army Navy game held on that weekend was related to the crowds.
... all the evidence points in one direction. The term originated in Philadelphia in the 50s or earlier and wasn't in common use in the rest of the country until decades later. And it did indeed refer to something unpleasant: the gigantic Army-Navy-post-Thanksgiving day crowds and traffic jams, which both retail workers and police officers dreaded. The retail industry originally loathed the term, and the whole "red to black" fairy tale was tacked on sometime in the 80s by an overcaffeinated flack trying to put lipstick on a pig ...