Based on the phrase,"once in a blue moon", you would think it didn't happen very often but it is not really that uncommon. Our moon orbits the Earth once every 29.5 days and we have 30 -31 days in a month (except Februay of course). So it's entirely possible to get 1 or 2 months with 2 full moons every 2 - 3 years. A great deal of weather folklore is based on the moon just because it's one of the most prominent objects in our sky. September's full moon is also called the Harvest Moon as well. As the air cools down from the Dog Days of August, it becomes drier. Less moisture refracts less light so it appears brighter. Farmer could then use it to harvest crops late into the evening. The intensity may also cause the atmosphere to scatter more of the blue wavelengths of light, much the same way it does during the daytime making the sky look blue, so the moon takes on a bluish tint.
The arrangement of craters and marias on the surface creating an image that looks like a man's face will continue to inspire people to give that big chunk of rock mystical powers for years to come. Being very bright and full more than normal just makes it more tempting to do so!