It is a bit tricky to decide whether a rare event should be classified as a black swan event or not. In general, in order to be classified as a black swan, it should meet the following four criteria.

1. First, once the event has happened, for observers it must be a mind-blowing surprise, for example, dropping the atom bomb on Hiroshima.
2. The event should be a blockbuster—a disruptor and a major one, such as the outbreak of the Spanish Flu.
3. Once the event has happened and the dust has settled, data scientists who were part of the observer group should realize that actually it was not that much of a surprise. Observers never paid attention to some important clues. Had they the capacity and initiative, the black swan event could have been predicted. For example, the Spanish Flu outbreak had some leads that were known to be ignored before it became a global outbreak. Also, the Manhattan Project was run for years before the atomic bomb was actually dropped on Hiroshima. People in the observer group just could not connect the dots.

4. When it happened, while the observers of the black swan event got the surprise of their lifetime, there may be some people for whom it may not have been a surprise at all. For example, for scientists working for years to develop the atomic bomb, the use of atomic power was never a surprise but an expected event.