The blue moon’s equally rare cousin, the black moon, will rise in Colorado’s skies Monday night, but you won’t be able to see it. While a blue moon is the second full moon in a month, the black moon is the second new moon in a month — or the third new moon in a season with four, instead of a typical three — according to Time and Date. New moons are fairly common and happen about once every four weeks, but they only appear twice in the same month about every 29 months, according to Time and Date.