The lyrics are presumed to refer to an English idiomatic expression: a blue moon is either the second full moon in a month or the third one when four full moons occur in one season of the year, which is a somewhat rare occurrence. If something happens "once in a blue moon" it happens almost never. The narrator of the song is relating a stroke of luck so unlikely that it must have taken place under a blue moon.
The lyrics are presumed to refer to an English idiomatic expression: a blue moon is either the second full moon in a month or the third one when four full moons occur in one season of the year, which is a somewhat rare occurrence. If something happens "once in a blue moon" it happens almost never. The narrator of the song is relating a stroke of luck so unlikely that it must have taken place under a blue moon.