That's a good story, but according to Wikipedia this etymology was invented after the fact and isn't the real origin of the term (which is somewhat obscure.)
I don't get what you're saying. It's not obscure at all that black ink denotes profits and red ink denotes losses - "in the black" and "in the red" are well-understood English phrases.
All I'm saying is that this particular meaning of black/red doesn't appear to be the origins of the name "black Friday", at least according to Wikipedia, but I really don't care about this enough to investigate any further.