We all know what a Blue Moon is, right? A second full moon in a month. A rare, but not unknown event. But a Black Moon?
A Black Moon is the opposite of a Blue Moon if you will. A second New Moon in a month.
Why am I telling you this? Well for those of us in the western hemisphere, we can see one this week, on the 30th of September. They are not that rare, coming along every 32 months or so, but as the New Moon is not visible, then a second one in a month is almost literaly an event that cannot be seen. The name, however is so evokative that it is just screaming to be used in a scenario.
Here are some seeds that come to mind.
The Black Moon, being an astrological/astronomical event, is a time where certain rituals or spells have a greater effect.
The Black Moon, being a type of New Moon, means the sky is darker, allowing for a better view of the stars. what stars are visible now that are not always so? What does this mean for rituals, or alien visitors?
The Black Moon is not visible, but under a black moon, certain beings that are not normally visible can be seen.
If not creatures, then writing, like that in the Hobbit of the writing that only appears Under certain phases of the Moon, so coudl writing, or symbols only appear at this time.
This particular Black Moon follows on from an even rares recent event, the first new moon of this month was a ring of fire solar eclipse, as the new moon also passed directly between the Earth and the Sun. What keeper Worth their salt could ignore such an alignment of astronomical events?
Those in the Eastern Hemisphere however, they're the Lucky ones, they get a Black Moon next month. That's right, the Black Moon over there coincides with Hallowe'en. Now, there's a scenario that just writes itself!
So all these ideas, where the upcoming event is fixed in time, and not to be repeated often give great deadlines, or indeed starting times for plots.