The first time the term "blue moon" appeared in English, some 500 years ago, it referred to something preposterous and impossible, used in a manor similar to the expression "Next you'll be telling me that the sky is green."
A couple hundred years or so later, the expression "blue moon" was used to mean never - as in "That will happen in a blue moon"; it meant something like "when pigs fly."
More recently, the term "blue moon" has come to mean something that happens every now and then, as in "Every once in a blue moon, I get a taste for pickles and ice cream." In addition, "blue moon" can refer to the second full moon in a given month. We're not sure whether the chicken or the egg came first on this one - it may be that one term sprouted from the other, or it may be that they sprouted all by themselves.
It would be pretty cool if I brought this up on the occasion of a blue moon, but the next one won't happen until 2012.
What's interesting about this to me is that the meaning of the term did an abrupt 180 at some point. It used to mean "never" and now it means "sometimes." How weird is that? I can only think of one other expression that has done that, sort of. "Fat chance" and "slim chance" ("fat chance" is used sarcastically, but has been used that way for so long that the term's not usually considered irony). I wonder how many others there are.