I love it when a client wants to go bold with a space and really go for it. When I suggest a neutral clients automatically think white but black is a great neutral and can add much more punch.
When used correctly, instead of making the room feel smaller black walls can give the sense of infinity. Modern furniture and antiques both stand out and it makes everything feel richer. Combined with warm textures and finishes it creates such a statement!
Just like any paint there are unlimited shades of black to you must select the right one. My favourite right now is Willow (CC-542) from Benjamin Moore. A warm black, it has enough brown in it to work with the historical colours in our winter light. I have used it on doors, accent walls and whole rooms.
Or how about using black tone-on-tone wallpaper to envelope a powder room with lots of reflective mirror and lighting – Beautiful!