There is actually a bit of controversy and confusion about this term, believe it or not. A series of misinterpretations in Sky & Telescope ended with the author of an article in the March 1946 issue attributing the 1937 Maine Farmer's Almanac as defining a "blue moon" the 2nd full moon within a calendar month. The actual definition stated that the 3rd full moon in 1 season was referred to as a blue moon but the above mistake stuck.
But why a blue moon and not, say, a red or purple moon? Occasionally, the moon might appear bluish due to suspended particulates in the atmosphere: dust, ash, and other debris. When Krakatoa erupted in 1883, tons of ash and dust were sent skyward and changed not only sunsets but the apparent colors of the sun and moon themselves for years.
Another factor is at work here. Before we get to the calendrical definition and even before the volcanic historical origin, the concept of a blue moon being something rare or ridiculously impossible was extant in western culture. A poem by the Bishop of Chichester in 1528 stated the following:
Rede me and be nott wrothe, For I say no things but trothe: Yf they saye the mone is belewe, We must believe that it is true.In other words, don't believe everything you're told.
Of course, the full moon seen in the sky right now is not a blue moon. Two full moons will occur this month (August 2012) which is why you might hear people talking about "blue moons" right now. But we'll have to wait until closer to the end of the month to see that "blue moon" -if you follow the more recent definition.
In Ball State University Libraries
Krakatoa: the day the world exploded, August 27, 1883 by Simon Winchester.
QE523 .K73 W56 2003B
The Indonesia reader: history, culture, politics by Tineke Hellwig
DS634 .I53 2009
Call number ranges for volcanoes: QE521.5-QE527.75