The Collaborative International Dictionary of English v.0.48:
blue moon \blue moon"\
The second full moon occurring in the same month; -- derived
from the expression once in a blue moon; as, we had a blue
moon on January 31, 1999, and another in March.
once in a blue moon very rarely; -- from the observation
that the moon rarely has a bluish tint.
Note: In 1980, the phrase blue moon was applied to the
second full moon of any month, because this phenomenon
is unusual. This new meaning for blue moon, a long
time, used primarily in the phrase once in a blue moon,
has become widely accepted.