Here's some information that shall explain what is meant by "blue" moon:

Calling the second full moon in a month a blue moon is a realtively new thing.

An older definition for the blue moon is recorded in early issues of the Maine Farmer's Almanac. According to this definition, the blue moon is the third full moon in a season that has four full moons. Why would one want to identify the third full moon in a season of four full moons? The answer is complex, and has to do with the Christian ecclesiastical calendar.

Some years have an extra full moon—thirteen instead of twelve. Since the identity of the moons was important in the ecclesiastical calendar (the Paschal Moon, for example, used to be crucial for determining the date of Easter), a year with a thirteenth moon skewed the calendar, since there were names for only twelve moons. By identifying the extra, thirteenth moon as a blue moon, the ecclesiastical calendar was able to stay on track.

"Blue moon" appears to have been a colloquial expression long before it developed its calendrical senses. According to the Oxford English Dictionary, the first reference to a blue moon comes from a proverb recorded in 1528:

    If they say the moon is blue,
    We must believe that it is true.

Saying the moon was blue was equivalent to saying the moon was made of green (or cream) cheese; it indicated an obvious absurdity. In the 19th century, the phrase "until a blue moon" developed, meaning "never." Like the common phrase "Whe pigs fly." The phrase, "once in a blue moon" today has come to mean "every now and then" or "rarely" — whether it gained that meaning through association with the lunar event remains uncertain.