A blue moon isn't blue and the popular definition is a mistake, a 1946 magazine misreading that everyone liked too much to fix.

The older, seasonal meaning is the third full moon in a season that has four. Sky & Telescope garbled it into 'the second full moon in a calendar month', that version spread through radio and trivia games, and it's now the one people use. Actually blue-looking Moons happen after big fires or eruptions put the right size of particle in the air.