Why is the Friday after Thanksgiving called "Black Friday?"

The moving of unprofitable state (red state) to profitable (black).,hence called as Black Friday.Due to large volume of purchase by consumers on Friday.,retailers move from unprofitable to profitable state

When a store make money they are said to be in the black. Thus the bussiest shopping day of the year is when they take in the most money and are in the Black as opposed losing money and being in the Red.