When is the next Moon with that Blue Aura and how rare is it?
It applies to the rare event of a second moon appearing later in a particular month. It is believed the expression "once in a Blue Moon" came out of the fact that when we look at the Moon it has a blue hue to its edges. People then applied "once in a Blue Moon" to signify a rare event. The blue aura occurs for the same reason that when ice crystals form in the air around us on a cold winter's night they take on an azure aura. That same affect applies to the moon by creating an azure aura around the Moon when we gaze at it.
Not all Blue Moons are Full Moons.
Blue Moon 2012 by NASA Goddard Photo and Video, on Flickr taken on August 31, 2012.