Hallo Eyspfeil,
Deine Deutung des blue moon konnte ich auf die schnelle nicht finden. Bekannt ist mir diese:
A blue moon is an "extra" full moon in years that have thirteen full moons. Most years have twelve full moons which occur approximately monthly, but in addition to those twelve full lunar cycles, each solar calendar year contains an excess of roughly eleven days compared to the lunar year of 12 lunations. The extra days accumulate, so every two or three years (7 times in the 19-year Metonic cycle), there is an extra full moon.
wikipedia
once in a blue moon = alle Jubeljahre
leo.org
Gruß franz_liszt