It's not rare, it's not actually blue, and it's certainly not gonna come with an orange slice on the rim.
But if you look to the sky tonight, you'll see it -- an actual BLUE MOON. You know, the astronomical phenomenon with which we describe rare things that are probably much rarer than blue moons. In fact, a blue moon -- commonly defined as the second full moon in a calendar month -- occurs about every two or three years, or a tad shorter than the eternity implied by the saying. For perspective, presidential elections are rarer.
If you're hoping to see a full moon that actually appears to be blue, you'll have to wait for particles from a dust storm or volcanic eruption to refract blue light, which is pretty damn rare, according to The New York Times. However, if you're a fan of sweet Belgian-style wheat beers, blue moons probably happen all the time.