The blue moon is something I've written about before, way back in 2011, and again in 2012. But just to re-cap, there are two ways of marking a blue moon. Many are familiar with the second full moon in a calendar month as being a blue moon. However, the older definition of a blue moon was the third full moon in a season with four, where most seasons only have three.
The two in a month definition actually came from a mistake printed in Sky and Telescope magazine, and it's stuck ever since. This makes sense, as most people are probably much more familiar with the every-day calendar than they are with astronomical seasonal cycles. (Here's an article from 2006, which explains in depth about the history of both terms.)
Now, the recent full moon was actually a seasonal blue moon. This lead to some confusion because, again, most folks know and go by the twice in a month definition, and that's fine. But I did run into one very special lady who was very... well, pissed off about the whole thing. She kept saying that she was sticking to the "old way" and that this was new internet hype and nothing more!
So, again. No. The twice in a month method is less than a century old. The seasonal method dates back to at least 1819, and is likely older than that. This isn't internet hype by any means, and a lot of us have been using this method for quite a while.
Now she did ask an interesting question of why the third full moon is blue, and not the fourth. Unfortunately, she later said this was a totally rhetorical question to show how little sense this method made, and I really should stop imposing information on her! Yeah, really.
The thing is... it actually does make sense. The third moon is given the blue name to keep the other lunar names matching up with their seasonal events as best as possible. The third moon instead of the fourth keeps the whole season from being "moved up" too far. That's really all there is to it. It's often why old luni-solar calendars stuck their occasional "leap month" into the middle-ish of their year, and not just at the end, so that seasonal festivals would still be around the same time as they should be.
Speaking of luni-solar calendars, this same woman also insisted that lunar and solar calendars have always been separate and it's only recently that Christians and neo-pagans have tried to mash the two together to try to fit in with the Gregorian calendar! Which shows an amazing lack of understanding about history. Luni-solar calendars are pretty darn ancient. They predate Christianity by a long shot. This is a calendar that is based on lunar months, fixed around a solstice or equinox. For example, the ancient Greeks fixed their lunar months around the summer solstice. This lady kept going on about the Celtic calendar, too, which is funny because the oldest known Celtic calendar we can reliably assemble is, you guessed it, a luni-solar calendar.
Really, let's be honest, the twice in a calendar month method only makes sense with the modern Gregorian calendar. If you go back to lunar based months, of course you couldn't have two full moons in a month, because one moon cycle is one month. Not that all these old calendars actually had something called a blue moon, or cared so much about the moons in their seasons, but again, it's not like this is some crazy new idea. Entwining lunar and solar calendars is really darn old school.
And I'm not trying to talk anyone out of using the twice a month method! It obviously makes sense to a lot of people, since again, the Gregorian calendar is one most of us use in our day to day lives! But it's always good to know the origins of what you're using in your personal practice. Especially if you're going to hang out in a public forum and try to call out people for being dumb internet-hype loving neo-pagans...