It actually uses more energy for most people's display devices to show black. It's because the screens are back-lit and it costs energy to block out the light r

It actually uses more energy for most people's display devices to show black. It's because the screens are back-lit and it costs energy to block out the light rather than let it pass through. Blackle was only really useful when CRT screens were around. It could be useful on OLEDs which are not back-lit but they have shorter life spans due to burn-in.