Microsoft Encyclopedia Encarta
One would think with the name blue moon, the moon would appear to be blue. However, this is not the case. The definition of blue moon is the second full moon in a month. The occurrence of this double moon is very rare, this is where the saying, "Once in a Blue Moon" comes from.
The moon has been reported to having appeared blue in the sky when the volcano Krakatoa exploded in 1883, its dust turned the sunsets green and the Moon looked blue in the sky all around the world for nearly two years. It has appeared blue at other times such as when enough dust has blown up into the atmosphere, or when a forest fire is large enough to throw smoke particles into the sky.