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

You're right, you're thinking about it wrong.

There's nothing at all racist about Black Friday and it is indeed always the day immediately after Thanksgiving.

I think one big reason for this is that many people get the Thanksgiving weekend off in the United States of America, so that Friday (today, actually, as I'm writing this on November 27, 2009) is a day off and the perfect day to get a head start on Christmas shopping.

Why is it "black" Friday, though?

Because it's the biggest shopping day of the year (which is pretty amazing if you think about it) and the day that many retailers and manufacturers become profitable on their books, essentially switching from "red" debt on an annual Profit & Loss statement to "black" profit.

Simple enough once you think about it: red = debt, black = profit and Black Friday is the day that companies hopefully switch to being profitable for the calendar year. This also means that if a company does well on Black Friday that the revenues booked through the rest of the year are profit and ultimately are the profit margin of the corporation for the year.

Shop! Shop! Shop! on Black Friday...

The following Monday has become known as Cyber Monday too, but as more and more people are moving online to shop anyway, I suspect that'll go away again in a few years. Oh, and worth mentioning is that there's a counter-culture Friday celebration called Buy Nothing Day.

Now, enough. Go shop, it's your civic duty. :-)