Black Friday, when referencing the SHOPPING day, relates to the first day that many businesses go from red to black in the books (many by floating their expenses on credit cards or other financial devices, knowing that they would make enough money during the holidays to cover their debts). Black Friday, as it relates to the stock market, has cousins Black Monday, Black Tuesday, and others (you get the point) to refer to devastating drops in the stock market, prices of commodities, war, famine, the loss of an eBay bidding war on a particularly desirable chip, etc.