LCDs as a transmissive display technology work by emitting a bunch of photons and then selectively filtering some out to achieve the desired color / pixel brightness. Any filtered photon is wasted energy, this is inherent to the display technology and is not limited to dark content, just exacerbated by it.

Given that, all things equal there is no way for LCD to equal the efficiency of a self emissive display, at best it's a question of when will the luminous efficiency of OLED exceed that of white/blue backlight LEDs... and honestly we're likely already at or past that point.