A blue moon has nothing to do with color. It is the term used to refer to the second full moon in a month. Originally it referred to the third of four full moons in a season. Either way, a blue moon is rare, giving rise to the phrase "Once in a blue moon" meaning something occurs very rarely, but is not impossible.
Even though the astronomical blue moon is not actually blue, it is possible for the moon to appear blue. This can (and has) happened when ash, dust or smoke fills the air and causes the moon to appear blue-colored. This can happen with a forest fire, but most commonly happens from a large volcanic eruption. Because the dust particles must be very small, and atmospheric conditions just right, it is rare to see a blue-colored moon.