Simply put, a Black Moon is the New Moon version of a Blue Moon, and is either: A month missing a Full or New Moon… this can only occur in February, as the lunar synodic period from like phase to phase is 29.5 days long. This last occurred in 2014 and will next occur in 2018. The second New Moon in a month with two. This can happen in any calendar month except February. And now for the most convoluted definition: the third New Moon in an astronomical season with four.