The belief that the second full moon in a calendar month is a "blue moon" has *not* been around for six decades, assuming that to be a reference to the Maine Farmer's Almanac of 1937. That stated (with no references, and no easily discovered antecedents) that the fourth full moon in a season was a "blue moon". (That wouldn't be all that rare, of course, but neither is a month containing two full moons).
This article was mis-interpreted (in a 1999 article in Sky & Telescope) to support the popular belief of the second new moon in a calendar month. A retraction was published a little later, but by then the damage was done. AFAIK no reference to the "two moons in a calendar month" meaning has been found in any source prior to 1985 - slightly less than two decades ago. I'd be happy to be proved wrong, but I'd want to see some evidence.
