Painting the exterior walls of your house black can be a sticking point when it's resale time. You might find black to be bold and impossibly chic.
However, not everyone will feel the same way. And, repainting the exterior is a bigger cost than just a black room on the inside of the house. The shade of black and how glossy it is are also big considerations. Finally, in warmer climates, it will cost more to cool a black house.