The black swan theory is a metaphor that describes an event that comes out as a surprise, has a major effect, and it often inappropriately rationalized after the fact with the benefit of hindsight.
Covid-19 crisis, for example, is not a black swan event. The term black swan is based on an ancient saying that presumed black swans did not exist. This saying became reinterpreted to teach a different lesson after black swans were discovered in the wild.
Nassim Nicholas Taleb developed the theory. There are three factors that make an event a black swan event.
Those are the disproportionate role of high-profile, hard-to-predict, and rare events that we cannot understand or expect. These are beyond the realm of normal expectations in history, science, or technology.
The second factor is the non-computability of the probability of the consequential rare events using scientific methods. And last, the psychological biases that blind people, individually and collectively.
After the black swan event, the reality is changed.