Oh never mind. I got too curious and found an answer to my own question (from Wikipedia):
The Farmers' Almanac defined blue moon as an extra full moon that occurred in a season; one season was normally three full moons. If a season had four full moons, then the third full moon was named a blue moon.
Recent popular usage defined a blue moon as the second full moon in a calendar month, stemming from an interpretation error made in 1946 that was discovered in 1999.[1] For example, December 31, 2009 was a blue moon according to this usage.