A blue moon is an extra full moon that appears in a subdivision of a year, either a fourth full moon in a season or, recently, a second full moon in a month of the common calendar. Metaphorically, a "blue moon" is a rare event, as in the expression "Once in a blue moon."
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 volcanic eruptions or exceptionally large fires leave particles in the atmosphere.