The phrase "once in a blue moon" used to mean a very long time but there was an actual calculation done in the "old days" for what a Blue Moon meant that is different than how it is determined now. In the old way, Seasonal Moon names were assigned near the spring equinox to make them agree with the rules that the church used in determining the dates of Easter and Lent. The beginnings of summer, falls and winter were determined by the "dynamical mean Sun" (how the Sun would move along the ecliptic IF it moved at a constant rate to produce seasons of equal length).
Usually, a season has three full Moons. If a season had a fourth full Moon, the third full Moon of the set of four was called the "Blue Moon".
However, a mistake in interpreting the old rule in a May 1950 issue of Sky and Telescope led to the current definition of a blue moon being the second full moon within a month. That doesn't happen that often. The full moon on August 31, 2012, was the last "blue moon" until July 2015.
If you want the details of the history of the Blue Moon and historical detective work behind discovering the Sky and Telescope error, see Sky and Telescope's "What's a Blue Moon?" article.
A truly blue-colored Moon can happen if there's a major volcanic eruption that puts ash up into the stratosphere. With all of the ash very high up in the air, the normally pale gray and white of the reflected sunlight can look bluer on our sky as we look at the Moon through the fine ash layer way up high.
While you're waiting for the next Blue Moon, take a look at this 3-minute video from the Center for Lunar Science and Exploration at the Lunar and Planetary Institute using imagery and altimetry data from the Lunar Reconnaissance Orbiter.
(Clicking the video will open it in a new tab or page.)
Finally, in honor of Neil Armstrong, his family has a simple request:
“For those who may ask what they can do to honor Neil, we have a simple request. Honor his example of service, accomplishment and modesty, and the next time you walk outside on a clear night and see the moon smiling down at you, think of Neil Armstrong and give him a wink.” [http://neilarmstronginfo.com/]