How to use the black screen
Go full screen
Press Go full screen and the whole display turns black with no menus or edges. The arrow keys switch to the other colors without leaving full screen. To exit, press Esc, double-click, or tap once and choose Exit; the mouse pointer hides itself after a moment.
On iPhone, Safari does not let a page take over the whole screen, so the color fills the browser window instead.
Check for backlight bleed
On LCD monitors and laptops, light from the backlight can leak around the edges and show as bright patches on a black picture. To see it, darken the room, set the screen to a normal brightness, and look straight at it from your usual distance.
Some glow that changes as you move your head is typical of IPS panels and is not the same as bleed, which stays in the same place. Phone cameras exaggerate both, so judge with your eyes rather than a photo.
OLED screens and dust
On OLED displays each pixel makes its own light, so a black picture has those pixels switched off. Any pixel that stays lit on black is stuck.
Black is also good for spotting dust and fingerprints, which catch room light. Check on white as well, since some marks only show on a bright picture.
Keep the screen dark
A black screen is handy when you want a display dark but still on: during a presentation pause, while a long download runs, or to cut glare in a dark room. Where the browser supports it, the page also asks the device not to dim or sleep while the color is up.
Save it as a PNG for a projector or wallpaper. For a full check of every color, try the dead pixel test. Everything runs on this page; nothing is installed or sent anywhere.