A blue moon is an astronomical phenomenon.
A full moon that is not timed to the regular monthly pattern is called a ‘Blue Moon’ being rare.
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. The extra days accumulate, so every two or three years, there is an extra full moon. Different definitions place the “extra” moon at different times. This information is an extract from Wikipedia.
The name does not refer to the moon’s color but reflects the rarity of the event and gives rise to the expression, ‘‘once in a blue moon.’’ The pictures were by Sanjay on my request. Thanks Sanju. 🙂