Okay, children, time to grow up now.
Dating back to the 1800's the term "blue moon" was used by the Farmer's Almanac to denote the appearance of a third full Moon in a season where four full Moon's will occur. Since there are normally 12 full Moon's a year (one per month), this works out to three per quarter (three month period). However, occasionally a quarter will see four full Moon's.
The naming arises because each Moon in a given season has its own name. This tradition of naming full Moons has existed for hundreds of years across many cultures. The names generally accepted today are those coming from the Farmer's Almanac.
For instance, during the second quarter of the year, the Moons are named Pink (April), Flower (May) and Strawberry (June). However, should another full Moon appear during this quarter it would be called, by default, a Blue Moon. Since the third of the four full Moon's in a quarter is called the Blue Moon, the order of Moon's in the second quarter of the year would be Pink, Flower, Blue and Strawberry.
Since Blue Moons only occur about once every three years, it became becomes a convenient measure of a long period of time, therefore giving rise to the cliche "once in a blue moon".
This info came from about.com