"Blue moon" does not refer to a blue moon, it refers to a full moon that does not appear in the calendar according to the rules, relative to the intercalation of the lunar calendar. There are only 12 full moons in most Western calendar years, roughly one full moon per month. But every 2 or 3 years there is an extra full moon, and this extra full moon is called a blue moon.

"Blue moon" is also used as a metaphor for infrequent events.

At 'All club' in Shanghai China 2021.