The term Black Friday was a internal term that only stores themselves used for a long time. However, the general public had an idea about it. Then somebody decided that "Black Friday" would be great as a marketing strategy and boom, there you go. It has become just more of a zoo/circus ever since.
As for where it originally came from. Over the years, retail stores, mostly the big department stores (Macy's, Gimbels, JC Penney, Sears, etc.), would usually run in a negative profit margin (which is represented in red in accounting sheets) during the course of the year. However, the day after Thanksgiving and always a Friday, which historically kicked off the Christmas/Holiday shopping season, created a big boost in sales and usually pushed the stores into positive profit margin (which is represented by black numbers), so that day internally was known as Black Friday.