Demented Smoka said

I'm pretty sure it's called Black Friday because of one contributing factors to the great depression was when the stock market crashed like two times in one week I think. I believe it was called like ack monday and black thursday.

Shout to Cypress Hill for Black Sunday though.  

I had always thought it was derived from accounting terms and practices. 

It doesn't really apply in many retail companies today but in the past, retail companies would be in the "red" before Thanksgiving. In the red is an accounting term meaning the company is operating at a financial loss. The day after Thanksgiving, they would then be in the "black" meaning they recouped their previous financial losses and were operating at profit. The term red and back has to do with accounting ledgers marking losses in red and profits in black ink. Accounting software and spreadsheets still use this standard.

While that sounds like it could be a reasonable explanation, the term "Black Friday" wasn't first used for the day after Thanksgiving until sometime between 1952 to 1962 (there is debate about when it actually started). For almost 100 years before that, "Black Friday" referred to September 24, 1869 when the stock market crashed 20% and commodity values dropped almost 50%. 

Even in 1966, when the first printed record of the day after Thanksgiving was called "Black Friday", it was from an article in a stamp collecting magazine called The American Philatelist that stated the Philadelphia Police Department used the name to describe the traffic jams and crowding in the downtown stores. The usage of the term by the police was common for about a decade before 1966.

The first positive use of the term "Black Friday" came from an article in The New York Times on November 29, 1975 referring to it as the busiest shopping day of the year.

The accounting reference didn't become thing until November 28, 1981 when the Philadelphia Inquirer published an article stating that as the reason for the name.

Other theories have also come out like that slave auctions discounted slaves after Thanksgiving but no historical evidence can be found that supports those theories.

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That is all that the Great and Almighty Intraweb revealed to me today when I requested an audience of its boundless wisdom on the subject. All hail the Great and Almighty Intraweb! May its unrestricted bias, misinformation, and opinions masquerading as "facts" live forever!