"A blue moon is an additional full moon that appears in a subdivision of a year: either the third of four full moons in a season, or a second full moon in a month of the common calendar.
The phrase has nothing to do with the actual color of the moon, although a literal "blue moon" (the moon appearing with a tinge of blue) may occur in certain atmospheric conditions: e.g., when there are volcanic eruptions or when exceptionally large fires leave particles in the atmosphere."
(Source: Wikipedia)