A "black swan" is a very surprising and unexpected event that often has profound benefit or consequences. This term serves as a metaphor based on the surprising discovery of black swans in Australia. Black swan is used in finance to describe an unexpected occurrence leading to drastic gains or losses.
Black swan has a somewhat similar meaning to the expression, "diamond in the rough." The rise of American patriots and the onset of the Revolutionary War, for example, was a black swan for the British, who expected allegiance from colonists. The popular play and movie, "Black Swan," tells the story of a delicate and innocent swan princess who contends with an alter ego driven by darkness and seduction.