A blue moon is not a typical, monthly lunar phase. Rather, it refers to an additional full moon that pops up during a season, or the third full moon that arise among the usual four in a given season. Usually, this extra full moon does not appear blue in color, unless there are some rare atmospheric conditions, like the presence of volcanic ash or smoke.
The term 'blue moon' was coined in the 16th century, when people wanted to express something that's not so familiar or occurs only once in a while. However, it was later believed that the blue moon referred to the rare occurrence of the moon turning blue. Nevertheless, it's now established and widely accepted that the blue moon has nothing to do with the actual color of the moon, but rather its infrequent occurrence.
Finally, a blue moon is an oddball event that happens every once in a while during a season or a month from the calendar. It's not your standard lunar phase, and it may or may not be blue in color, depending on the surrounding atmospheric conditions.