There are a couple different definitions for a blue moon, both of them kind of rare, and neither of them have to do with color of any sort. You’re on the Sound Beat.
So the most common definition of a blue moon is a second full moon in a month. As the moon orbits around the Earth every 29 days or so, those extra days in a month accumulate. As such, every 2 or 3 years we get an extra full moon slipping into a calendar month. There’s also another definition for a “seasonal” Blue Moon: the third of four full moons in one season. And believe it or not, this definition predates the other.
You’ve been listening to the incomparable Mel Torme backed up by Pete Rugolo and his orchestra on a Capitol 78 from 1949. Blue Moon was never a rarity when Mel sang: indeed, it was one of his signature songs.