From Wikipedia:
"The most obvious meaning of blue moon is when the moon (not necessarily a full moon) appears to a casual observer to be unusually bluish, which is a rare event. The effect can be caused by smoke or dust particles in the atmosphere, as has happened after forest fires in Sweden and Canada in 1950 and, notably, after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years."
I think it would be a startling thing to look up and see the moon in any other way than I have seen it for 40 years. Cool.