A blue moon is the second full moon in a calendar month.
A seasonal blue moon is the third full moon of a four full moon season. Normally a season has three full moons. Every 2-3 years a season will have four and the third one is called a seasonal full moon.
A season is the period of time between a solstice and an equinox, or an equinox and a solstice. These are typically three month periods, which is why they usually have three full moons.
The next full moon is again in Aquarius and will be a seasonal full moon.