The black swan has a particularly unique position in literature and metaphors, owning to a belief centuries ago in Europe that all swans were white. The term came to stand for rare, hard-to-predict, high impact events. If you're interested to find out more read here.
By the way, if you are a regular reader, you may have noticed that I am making some changes to the web site. Unfortunately, Blogger ate some of my widgets. Bear with me. All will be back soon.
-- steve buser