Typically, a Blue Moon is defined as the second full moon that occurs during a calendar month, but the full moon this Saturday May 21 will be the only full moon of May 2016.
On the page for August 1937, the "Maine Farmer's Almanac" gives the calendrical meaning for the term "Blue Moon."
And that extra full moon also meant that one of the four seasons would contain four full moons instead of the usual three.
Pruett ultimately came to the following conclusion: "Seven times in 19 years there were — and still are — 13 full moons in a year.
It's unfortunate that Pruett did not have a copy of that 1937 almanac at hand, or else he almost certainly would have noticed that his two-full-moons-in-a-single-month assumption was wrong.
For instance, in the aftermath of the massive eruption of Mount Pinatubo in the Philippines in June 1991, there were reports of a blue-colored moon and even a blue sun worldwide.