A blue moon is two full moons in one named modern month. Do to the months having 28 to 31 days causes more days in a calender month than a lunar cycle.
Many of the calenders before the sexagesimal type we use today were based on a thirteen month yearly calender following the moon. Each lunar cycle in one months time. Therefore there is no magical base in history for the blue moon or the black moon (two new moons) theory except for the magical power you place on it due to it being special according to the modern calender.
Once in a blue moon I feel like an old book of interesting info that most don't give a hoot about. Alas, poor Yorick! I knew him, Horatio, a fellow of infinite jest, of most excellent fancy.