A black mirror? Much like a black Google blackle.com , without hard evidence, my guess is the energy savings is almost none.<p>Since my first phone with a background c. 2010, I have never changed the background from whatever the default is. I keep waiting for something important enough. I should probably reflect on that at this point.
If it's a OLED screen, than this is a pro tip.<p>Never did it differently.<p>But with non-OLED, this isn't useful to much. Pull the brightness down. Which also a pro tip for OLEDs :)