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I think you are quite wrong there.
It is a fact that the LCD screen pixel structure is only about 5% efficient at letting the light through when fully on, that means 95% of whatever the light source is either CC tube or LED is blocked and is therefore turned to black body heat. If the screen is black it would just be 99.9% instead. To save power means controlling the light source power.
There was/is a company called Unipixel that had a LCD alternative that claimed 60% efficient light cell based on a neat MEMs opto capacitive structure. They even licensed it to Samsung in 2009, not a squeek since though. When driven by side lit LEDs, its power use would be far below todays LED LCD panels which are mostly already good enough.