The thing that makes OLED screens so energy efficient is the only areas of the screen receiving current are those displaying a "color". Black areas are simply unpowered locations/pixels. It's why you see OLED phones like the Samsung Galaxy generally with black backgrounds and light lettering, more energy efficient. Regular LCD screens are backlit, usually by LEDs, which are always on, and the screen "simulates" black by "closing" those pixels to obstruct the light source. As the LEDs are on when the screen is active, regardless of what's being displayed, they keep consuming power.
That, and good OLEDs have really great color rendition, although sometimes it can be almost over the top. But OLEDs are really the only way to get a true, pure black out of a screen, although LCDs have gotten way better.