I'm not sure that's what he's getting at. He's just saying that "Once in a Blue Moon" is now a common expression that means something rarely happens. But in the world of astronomy, a "Blue Moon" (the second full Moon in a month) is not really a rare event. It happens every 2 to 3 years. And it's also just a normal calendar thing since a moon cycle is about a day shorter than a calendar month.
I'm not sure that's what he's getting at. He's just saying that "Once in a Blue Moon" is now a common expression that means something rarely happens. But in the world of astronomy, a "Blue Moon" (the second full Moon in a month) is not really a rare event. It happens every 2 to 3 years. And it's also just a normal calendar thing since a moon cycle is about a day shorter than a calendar month.