A black swan event is an occurrence (such as the collapse of the Soviet Union, the sinking of the Titanic, the conquest of the Aztec Empire by a tiny band of Spanish conquistadors) that was completely outside the realm of expectations.
I believe Bekele's performance today was a black swan event of sorts, since basically nobody expected him to scare the WR.
Another T&F black swan was Billy Mills' surprise victory in the 1964 Olympics 10000 meters. What others can you think of?