The 'Blue Moon' does't have anything to do with color. The second full moon that occurs within a given calendar month is called a Blue Moon. An ancient tradition calls the fourth full moon, in a season, as Blue Moon. Each of the four seasons in a year will have three months and usually three Full Moons. Each of these 12 Full Moons, of a year, has its own name in folklore, like "Harvest Moon," "Hunter's Moon" etc. When, however occassionally,a season got four Full Moons, it was called "Blue Moon", as there was no name for this occassional Full Moon. "Harvest Moon" is the Full Moon of Harvest time or more exactly, occuring just before the autumnal Equinox on about September 23. As the continuance of the moonlight after sunset was useful to farmers in northern latitudes, who usually harvest their crops, the name stuck. The Full Moon following the Harvest Moon, which exhibits the same phenomena in a lesser degree, is called the Hunter's Moon.