A moon is called a "blue moon" when it is the second full moon in a month. The moon will wax to its full phase at 9:58 a.m. EDT (1358 GMT) today, bringing August's full moon count to two (the first one occurred Aug. 1). Two full moons won't rise in a single month again until July 2015.
But don't expect tonight's full moon to actually appear blue, unless you're peering through a thick haze of volcanic ash or forest fire smoke. "Blue moon" is not a reference to the satellite's observed color.