Black Friday is actually an economic thing (day after Thanksgiving in US) the day the companies go into the black/ start showing a profit for the year.
Oops, you're right, I should have researched that a bit...
Still, apparently the origin of the name seems to have more to do with the idea of a "black day" (a day when a disaster occurs), according to Wikipedia:
> The earliest evidence of the phrase Black Friday originated in Philadelphia, where it was used by police to describe the heavy pedestrian and vehicular traffic that would occur on the day after Thanksgiving. This usage dates to at least 1961.
You were thinking of Good Friday.