Excerpted from Christian Science Monitor:
For most people, it will just mean a full pie in the sky Thursday night. A blue moon is simply the second full moon in month. It doesn’t actually have anything to do with color at all.
According to NASA's website, the term blue moon was "used in much the same way we use the term 'harvest moon.' There were twelve names for full moons, one for each month, and the name blue moon was used in years which had 13 full moons."
But in 1943, Sky and Telescope Magazine erroneously wrote that the second full moon in any calendar month was called a blue moon. The label stuck and is still used today.
Interestingly enough, this is not where the expression “once in a blue moon” comes from. According to NASA, that phrase is believed to have originated in 1883 after the eruption of Indonesia’s Mount Krakatoa. The volcano put so much dust in the atmosphere that the moon actually looked blue in color. The event was deemed so unusual the phrase “once in a blue moon” was coined.