Slate Overcast
It would basically be Slate showers,but slightly darker and without the rain.
Slate Midnight
This slate map would have a dark colored grass for ground(No grass emitter),and a cloudy skybox with a full moon.
It would also be dark, but of course moonlight would add an eerie effect on things.
Good use for this would proably be halloween builds or other spooky or creepy builds.
Just an idea would be nice for certain builds.