Why was the day after Thanksgiving originally called 'Black Friday' before it became associated with holiday shopping?

Black Friday has become a global shopping phenomenon that kicks off the holiday season. But this retail innovation has an interesting history that many pop culture enthusiasts might not know. Test your knowledge about how this massive shopping festival got its name and became a cultural touchstone that's referenced in movies, TV shows, and social media every year.

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Because of the massive traffic jams and chaos that occurred when people flooded downtown Philadelphia in the 1960s
Because retailers traditionally moved from operating at a loss ('in the red') to profitability ('in the black') on their accounting books
Because of a stock market crash that occurred on a Friday after Thanksgiving in the 1950s
Because stores would open before sunrise when it was still dark ('black') outside