It's from old records. When you were not making a profit, losses were recorded in red (as in "in the red"). When you were making a profit you wrote in black. Black Friday is when you could expect any losses to be made up for the year for a lot of retail companies.
It's definitely because that one day puts their books in the black. Used to work retail myself. Now I avoid everything except the feed store and the grocery store on that day (and frankly, for most of December)
The businesses run red for 364 days and then all of the sudden one day puts them in the black. I don't buy it Scooter. I won't help them. Have a Merry, Merry $$mas.