Before Australia was discovered, the British had only ever seen white swans. So firm was this belief (that the only kind of swan is white) that there were even common sayings about this idea, such as “you’d sooner see a black swan than…” (meaning something is impossibly rare).
The discovery of the new world put an end to this delusion. In finance, a black rare swan is used to refer to impossibly rare market phenomena that have catastrophic effects when they occur.
It is essentially incredibly challenging to predict before it happens but seems evident after the fact.
The 2008 crisis is a perfect demonstration of what a black swan is. The term was popularized by Nicholas Nassim Taleb in his book The Black Swan.