Once in a blue moon means happening extremely rarely. Only once in a blue moon, we meet once in a blue moon, once in a blue moon at best.
A blue moon is a second full moon in one calendar month and happens about every two and a half years, so the phrase names something rarer than it actually is. The moon does not turn blue.
1phraseonce in a blue moonHappening extremely rarely.
“My brother lives abroad, so we only see him once in a blue moon.”
once in a blue moon is a rarity.
From blue moon, first recorded in this sense in the nineteenth century.
See blue moon. First use appears c. 1833. See citation below.
Etymology from Wiktionary, CC BY-SA
Practised in Idioms