It all depends on what the resting mode of an LCD panel is, black or white. That is, whether or not the crystals default to passing light up through the second polarizer, when _not_ activated by power.
As it turns out, while older type (e.g. TN) LCDs do usually default to a white resting mode (meaning it takes slightly less power to be all white)...
... IPS LCDs such as the display used in the iPhone default to a black (okay, really dark gray) rest mode. Thus, it takes more power to activate the crystals to let the light through to make a white screen.
So, a dark mode will use less power on both LCD and OLED iPhone displays.