What is a blue moon?

A blue moon is what people call the second of two full Moons that appear during the same month. A second full Moon can appear within the same month because full Moons occur about every 29.5 days. So if a full Moon occurs in the first day or two of a month, except for February, a second full Moon can occur during one of the last days of that month as well. It doesn't actually look blue.