What a black screen shows you that white does not
White finds dark faults. Black finds bright ones, and they are the faults that tend to be worse. A stuck pixel, lit permanently in red, green, blue or white, is nearly invisible against a bright desktop and unmistakable against pure black. So is light leaking in from the bezel, the cloudy patches that plague cheap VA panels, and the pale glow across a corner that tells you an IPS monitor is doing what IPS monitors do.
It is also the correct screen for cleaning a glossy panel. A dark screen reflects, so you can see the smear you are chasing as you chase it, instead of discovering it later when you open a white document.
Reading what you see
- Small bright dots that stay in exactly the same place: stuck pixels. Try the stuck pixel fixer before you claim a warranty.
- Light creeping in from one or more edges or corners, worse in a dark room: backlight bleed. Common, and only a fault when it is bad enough to see in normal use.
- A broad, soft, uneven haze across the middle of the panel: clouding, also called mura. Harder to live with than edge bleed because it sits where you look.
- A glow in the corners that shifts or disappears as you move your head: IPS glow. It is a property of the panel technology, not a defect, and no replacement unit will be free of it.
- A blurred white patch that does not move and is also visible on other colours: pressure damage to the diffuser layer, which is physical and permanent.
Do it in the dark, but not in the pitch dark
Backlight bleed looks catastrophic in a completely black room at full brightness, and almost every monitor ever made would fail that test. Judge it at the brightness you actually use, in a room with the lights low rather than off, after giving your eyes a minute to adjust. If you cannot see the bleed while doing normal work, it is not a fault worth returning a monitor over.
Black on OLED is a different thing entirely
On an LCD, black is the backlight still shining through a closed shutter, which is why bleed exists at all. On an OLED, black is the pixel switched off, emitting nothing. If you open this page on an OLED phone, laptop or television and the screen simply vanishes into the bezel, that is the panel working correctly. It also means a black screen uses almost no power on OLED, which is why dark mode saves battery there and saves none at all on an LCD.