When a full moon is not enough to meet a writer's need for the symbolic, they can use the blue moon for it's rarity value. They didn't coin the Stock Phrase "once in a blue moon" from nothing now did they.
Now there's been much confusion over what constitutes a "blue" moon. According to some usages it means the second full moon in a month, or to the third full moon of a season with four full [...] [tvtropes.org]