Posted by John Fletcher on September 13, 2004
The following appears under the Blue Moon heading.
Another possible derivation is from The Maine Farmers' Almanac. When there were two full moons in a calendar month the first was printed in red, the second in blue.
The Maine Farmers' Almanac defined the blue moon as the third moon in a season that had four full moons.
see http://www.obliquity.com/astro/blue-st.html for full details