This phrase is commonly known and used in the finance world. It is a highly negative occurrence that is challenging to predict. In simple terms, black swan events are not only unexpected but also unknowable. This term was made popular by the former Wall Street trader called Nassim Nicholas Taleb. The Black Swan Event aids you to understand whether the altcoins are uptrend or downtrend against BTC.
When BTC increases, the alts lose BTC value, and when BTC decreases, alts gain BTC value. It, therefore, means that you will want join Bitcoin when the dominance has an uptrend and join alts when the dominance is downtrend.