There are two types of blue moon and neither refers to the moon's visible colour. The first one is the full moon that appears for a second time in the same month. The last one of this kind appeared on August 31, 2012, with the next one scheduled for July 31, 2015.
The second one is the third full moon of a season where the season contains four full moons instead of the usual three. The moon I saw today in the early morning is one of such kind. The next one is to show up on May 21, 2016. You can clicked the cropped image above to see the full-resolution version in its 2700x2700 pixel glory.
Settings: Canon SX50 HS - 1200mm (w/ 2x Digital TC) - ISO 80 - 1/400s
Time: 2013-08-21 01:00AM KST