There are many stories about the phrase ”Black Friday”. Here you can read about two of them.
One story is that it was first used by the police in Philadelphia around year 1961. It described the chaos when crowds of people came into the city the day after Thanksgiving. The police had to deal with traffic jams, shoplifting and people who got hurt from the crowding.
The rest of the country didn’t use the term until the late 1980’s. Then the stores would use the term to describe the point in the year when they could finally make a profit. The numbers went from red (losing money) to black (making money).