A Blue Moon is 2 full Moons in one month of the Gregorian calendar. So the May 21 full Moon in Sagittarius is not a Blue Moon according to that definition.
But an older interpretation of a Blue Moon is 4 full Moons during a solar cycle (between an Equinox and a Solstice, or between a Solstice and an Equinox).
Now is the 4th full Moon between Spring Equinox on March 19 and Summer Solstice June 20 in 2016, so this does qualify as a Blue Moon in the older astronomical meaning.
Happy full Moon!