A blue moon usually means the second full moon in a calendar month. The Moon does not normally turn blue; the phrase is about timing.
Why blue moons happen
The lunar cycle is about 29.5 days, while calendar months are usually 30 or 31 days. Sometimes that timing allows two full moons to fall in the same month.
Another older meaning
In some traditions, a blue moon can mean the third full moon in a season that has four full moons. Both meanings refer to an extra full moon in a calendar pattern.
A blue moon is different from a supermoon, which is about the Moon's distance from Earth.