Why is it called Black Friday?

I am not sure why they call it "Black Friday?"
Kim and I went Christmas shopping on "Black" Friday and all we ended up was in the RED!
Someone said it is called Black Friday because it is the day of the year that retailers in the United States make the money that puts them in the black on their balance sheets for the year. But with all the really great deals and offers they put out there for us consumers to purchase, one wonders how they can make any money on some of them.
Big screen t.v.s slashed to less than half what they normally cost the rest of the year. The same goes for countless other items on sale that day.
It makes me wonder. If that day puts them in the black, what are they charging the rest of the year? If they are making huge profits on Black Friday, they must be making outrageous profits when the prices are much higher the rest of the year!
The other thing that bugged me this year is that all week there were pre-Black Friday specials. Then Thursday (NOT FRIDAY) evening many of the stores opened up their Black Friday sales. It seems Friday is more than 24 hours long! Maybe next year it will be Black Friday month!
–Until then,
Ken