Black Friday used to be the day when retailers became profitable for the year, but this is no longer the case. Black Friday has become so ridiculous that most major retailers now actually lose a lot of money on the day, but they don't have the guts to do away with the concept, and in fact continue to try to one-up each other.
It's supposed to be a big day, financially, for businesses. They usually have out a lot of Christmas items, and some new or big-selling electronic stuff. I've heard Cyber-Monday, the following Monday, is also supposed to be a big-selling day online.
Black Friday was used to compare with the heavily stressful and chaotic 1929 Stock Market crash, called "Black Tuesday." But in terms of shopping, it is used to describe the extreme traffic and crowds among the streets and stores on the day after Thanksgiving: This is when the Christmas sales officially begin.
Bus drivers and taxicab drivers also use this name to describe their headaches that they would receive on that day from the people rushing to get to the store sales on that day.
It refers to a financial term. Most stores and shops operate in the negative and are losing money for the entire year. Then the first day of the Christmas shopping season, their sales jump up enough so they make money or, as they say in accounting, they are "in the black."
When I was in college in the 70's, the Friday after Thanksgiving was the day my mom used to take my brother and I shopping to get ideas of what we wanted for Christmas before we headed back to school. There were good sales and that's also when my brother & I would buy gifts for our family members. Most of our friends did the same thing with their families. It was crowded but not nuts like it is now. They didn't have the crazy competitive sales and nobody went early to get in line. You couldn't pay me any amount of money to go shopping on Black Friday now!
It's true that this is the day which many businesses hope will put them "in the black", however, if you work retail, it's black because its the hardest day of the year to work. Some of my friends will have to work 12, 14, or as much as 18 hours, on their feet, probably with no breaks or meals, and, unfortunately having to be unfailingly polite to customers--excuse me, "guests"--who are demanding, surly, rude, and generally unpleasant. For my friends, it's a black day, indeed.
"Black" probably means so many crowds all you see is black and also one day for big business to sell stuff cheap but still making a lot of money. Then that Saturday is when small business are recognized.