Actually this is a little "not true".
If you are using an old CRT monitor then yes this will save energy.
If you have a newer LCD it takes more.
This is because the background light is always on. The LCD is above this and is changing colors to fit what ever you are watching.
White all pixels are off letting all the light through.
Black all pixels are on max, taking max energy to "block" the background light.
Official Google Blog: Is black the new green?