Seems to be varying definitions of a Blue Moon but it looks like the original inception of the term referred to it as the third full moon in a season where there were 4 and an intrpretation error in 1946 changed the label as it is today.
http://en.wikipedia.org/wiki/Blue_moon
"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 would be a blue moon according to this usage"