(It goes back to the days when small merchants had their own in-office accountant. And the norm was to use black ink when your books showed your business was making money, and red ink for the times when you were losing money.)
The first Friday after Thanksgiving was traditionally the day when people started their Christmas gift shopping. A LOT of shopping. So much shopping, in fact, that the stores who weren't doing all that well in the middle of the year, could count on increased profits for the end of the year. And as a result of that...the ACCOUNTANTS (remember them?) COULD SWITCH FROM USING RED INK TO BLACK INK WHEN BALANCING THEIR STORE'S BOOKS.
And because of that, the Friday after Thanksgiving started to be called Black Friday.