this interesting discovery today. It turns out that there is actually something called a "black swan theory," based on the 2007 book of the same name by the Lebanese-American writer Nassam Nicholas Taleb. It turns out that until the 17th century, many Europeans did not know that black swans existed as a unique species of swans. In other words, saying "a swan is white" was akin to saying "the pope is Catholic" -- as in, isn't that obvious? (No offense intended toward Catholics here.) It took a voyage to Australia for Europeans to realize that their scientific observations about all swans being white, due to the lack of any black swans to observe, were limited to their geography: in Australia, cygnus atratus was alive and well.
With this in mind, Taleb developed his black swan theory, which is essentially a metaphor for people being surprised by something they assumed not to be the case. The theory goes on to claim that after the discovery people attempt to rationalize and explain their ignorance. They also respond with increased care to avoid future, similar, errors. (Taleb discusses the events of 9/11 in terms of the black swan theory.) This is ultimately an exploration of probability and the way that people react when the seemingly improbable becomes not only probable but also real. (The reference to the black swan in this theory dates back to the earlier "black swan problem" of philosophers, this problem being one of induction.)
So, they're you go. In the ballet Swan Lake, the Black Swan character (Odile) is presented as the "evil" one: she is the daughter of the sorcerer Von Rothbart, and she has been transformed to look like the White Swan Odette -- ultimately, she wears the black costume because the same dancer performs both roles (and how else are you to tell them apart?). The black is a metaphor for her character, of course. But in philosophy and statistics, black swans perform a much more interesting (in my opinion...) role of defining a standard of probability.
They also have those great red beaks. That has to count for something.