The name comes from the fact that many businesses make gains for their yearly profits, putting it in the "black" ink as opposed to the red ink, which they often operate on in the months preceding Black Friday.
Another theory or application of the name is that in a sense it is the exact opposite of Black Tuesday, in that the retail economy booms as opposed to collapsing as it did in 1929 on Black Tuesday.
Dad - "Don't forget your fifth prayer tonight, Akshay. We must respect Allah so that we get the coupon for the plasma TV special. Alahem-ahew-akbar."
Son - "What does alahem ahew-akbar mean father?"
Dad - "That we may be so fortunate as to also get the coupon for free delivery."
Really, this is a stupid time to have "black friday" because you're so upset at your family for making your thanksgiving hell that you get them crap for Christmas and later feel guilty and regret it and have to go out and buy them something else the week before Christmas, when the stores are crowded with other people doing the exact same thing.
Black Ladie 2-"Dam Dats what I call a m'f-n deal! So dats why they call it Black Friday!"