From the Now This website:
Black Friday used to be a term to describe pedestrian and car traffic In the 1950s, police in Philadelphia always saw a ton of chaos the day after Thanksgiving Hordes of people would head into the city that Friday ahead of the huge Army-Navy football game held annually on the Saturday after Thanksgiving. Philly police called that chaos Black Friday. They could never take that day off and worked extra shifts because of how congested it was.