As others mentioned, if we can define the type of event that we fear, then it is not a "black swan" because we would not see it as a possibility beforehand, though we may see it as obvious in hindsight. Of course there always seem to be some people who saw the possibility of a certain shock beforehand, but there was no way they would have known when/how it would exactly play out, and it's unrealistic to expect someone could correctly predict several of these events throughout their lifetime even if they got lucky or had the rare expertise to be able to predict one of them.
The existence of Black Swans implies that financial markets, global events, etc., do not follow a normal probability distribution with symmetric probabilities of positive and negative events. The negative left tail is fatter than the positive right tail of the probability distribution. For example, if looking at S&P500 6-month price changes vs. their long term median 6-month price change, there has been a greater occurrence of prices falling short of their average by 30% or greater vs. exceeding their average by 30% or more (data going back to 1928). Even worse, a 30% shortfall requires a 43% recovery to get even whereas an unexpected 30% windfall would only take a drop of 23% to wipe out.
For a passive buy & hold investor, the solution is psychological. Don't worry about such risks and know that this negative tail risk dissipates over longer periods of time - invest for decades and don't worry about a bad 6 month run that couldn't have been predicted. For a non buy & hold investor or trader, either invest/trade in such a way that negative tail risk doesn't matter to your income flows, or find a way to insure against it, or find a way to alter the expected distribution of your returns such that they don't have the negative tail skew that buy & hold investors face.
