The Full Moon of July 30-31, 2015 is the second Full Moon in the month of July, which is what some have come to call a "Blue Moon." Oddly, it is not blue and contrary to opinion, it isn’t particularly unusual.
Defined as the second Full Moon in a given month, "blue moons" are not that rare. They happen, on average, every couple of years. During my lifetime, there have been more than 30. On a scale of rarity, that’s not exactly up there with the number of times I have won the lottery or been elected president or won the Nobel prize. It is vastly more common than that.
And even by that definition, the Moon really isn’t blue anyway. The true color of the Moon does not change, although at times it can appear blood red, blue or sometimes other shades.
The color reference is just an oblique reference to the idea that the event was thought to be somewhat unusual. It is relative, but a true "blue Moon" – one the actually appears blue — is in fact much more rare and unpredictable.
There are a few atmospheric circumstances, having nothing directly to do with the Moon itself, that can cause the Moon to appear blue. One of those relates to smoke in the atmosphere. In September, 1950, smoke from fires in British Columbia, made the Moon (and Sun) appear blue for many observers in Canada and even the UK. Here is a report from the CBC about the 1950 "blue moon."
So as popularly defined, Blue Moons are not that rare. But by a definition that requires a Moon that appearing truly blue, they are somewhat more rare, and certainly not predictable.