In all these years I cannot recall seeing a "blue moon". Of course the moon is not really blue, but it can adorn the appearance of blueness [and other colors]--all based on the particulate matter dispersed in the atmosphere such as volcanic eruptions or forest fires. Well, linguistics and science are at play here and from a linguistic perspective it is rare and thus the coined term "a blue moon"--generically, something [an event] happening rarely. But, it still could be a physical phenomena of a moon appearing "blue" in color.
Cool moon map...