If I had a guest room I would love to go bold with it and paint the walls dark/black. I don't know why the guest room, I guess because it's a room to go a little crazy with and make an impression on your invités without committing to dark walls in your own bedroom.
Black walls can be 100% amazing or be a total disaster as they won't reflect the light so here are few things to consider if you thinking of going dark!
1. The size of the room
You might think bigger room will accommodate the dark walls better but it is not always the case.
It's easier to create a dramatic effect in a smaller space, bigger room can look oppressing and might be more difficult to keep it looking lively.
Black walls will absorb the light, make sure you have extra artificial light like bedside and floor lamps to keep the room cozy.
Black walls will look great with metallic surfaces, pastels, white, patterns. You will need to make sure the decor flows through the room and the accessories add life and light so it doesn't look dull and sad.
Here are some great examples of dark room that works!