Changing the brightness of an LCD screen does not affect the contrast of the emitted light. The lowest possible black is some percentage of the backlight bright

Changing the brightness of an LCD screen does not affect the contrast of the emitted light. The lowest possible black is some percentage of the backlight brightness, not a fixed value. You can see this by changing the brightness while displaying a pure black image. LCDs do also reflect some ambient light, so turning down the brightness will reduce the contrast slightly in most environments, but many LCDs have anti-reflective coatings to mitigate this. LCD typically has higher maximum contrast than e-ink.