If all the lights are out, then any light needed to see what is going on should feel like it is coming from outside the room, whether moonlight or streetlight or light leaking in from another room through a crack in the door or something.
Besides light coming through any windows, additional light to see something, if needed, should be very soft and very dim so as to feel like ambient bounce light in the room, the light from outside bouncing around the room.
The color is up to you, you can justify anything from the outside -- could be cool moonlight or LED streetlamp, or blue-green mercury-vapor industrial or cool white fluorescent, white tungsten, orange-yellow sodium vapor, colored neon, etc. If this is during a city-wide power outage then it would have to feel like moonlight.
"Dark" can either be high-contrast with black shadows and a few brighter highlights or low-contrast with very dim highlights.
How bright the lighting is just depends on what working f-stop do you want to shoot at and what levels are practical to achieve, plus if you have to balance your lighting with sources that you can't control how bright they are in the frame, like a candle or a small flashlight or something.
But unless you are balancing to a practical source that is naturally dim so your lighting has to be very low in level to match, if you just want a dark scene, there is not necessarily a need to use very weak, low light levels -- after all, people have made sunlight look like moonlight, and that's quite a lot of light. Also, you might want to work with enough light that you are using an ASA rating on the camera that gives you a noise level that you like.