It literally meant the day stores would go into the "black" because many stores would be in the red for the year overall until they sold so much on that one day they'd turn a profit for the year or go into the "black" on their accounting.
Black friday is a term for a day when something horrible ocurs, the term was applied to the day after thanksgiving by police in an american city because of the difficulty in maintaining crowd control and it caught on.
While black Friday is a reference to a single day in history. It is also a reference to the annual shopping for Christmas. Which has also changed. It really was the start of the shopping season but alas, you cant stop change