In simplistic terms, a 'black swan' event occurs when everyone is looking to their left, but then get hit from the right.
Over my investing career, I've witnessed three black swan events:
1) In 1999, most investors thought the Internet had created a new paradigm for revenue-less companies. Since cash flows were nonexistent for many Internet companies, valuations were based on eyeballs and untapped potential. Then, seemingly out of nowhere, Internet stocks collapsed.
2) In 2005/6/7, most people thought houses were a sure thing. Everyone was buying and flipping "McMansions", and assets, in general, benefited