The term "blue moon" doesn't refer to the color of the moon. It refers to a second full moon in a calendar month.

Since the lunar cycle (the time it takes for the moon to go through all its phases) is about 29.5 days, and a calendar month can be 28, 29, 30, or 31 days, it's possible to have two full moons in a single month.

Therefore, a blue moon occurs roughly every 2.7 years.