[foota]

It depends afaict. OLED screens have a per-pixel light, and they turn off pixels to make black. LCDs have a single large backlight and pixels that the light shines through and they can change color (but not turn off) so in that case they turn as opaque as possible, but don't completely block the light.

[Ekaros]

There is also things like microled. Which means that there is bunch of small(bigger than multiple pixels) lights that turn on and off as needed.

[thayne]

And in a CRT the electron beam turns off when scanning over a pixel.

[account42]

Black pixels in an OLED still reflect some light so their are not completely black either.