A "Blue Moon" is the second full moon in a calendar month, occurring roughly every 2.7 years. Despite the name, it doesn't appear blue in color.
The term "once in a blue moon" meaning something rare comes from this astronomical phenomenon. However, the name might actually derive from the Old English "belewe" meaning betrayer - the moon that betrays the typical pattern.
Astrologically, blue moons are considered powerful times for manifestation and completing projects that require extra lunar energy.