OLED displays have the highest contrast of any display panel. Blacks are pure black because OLED, which stands for Organic Light-Emitting Diodes, can switch off individual pixels.
iPhones with OLED screens should be handled with a bit of care to avoid image persistence or screen burn-in, if you treat your new iPhone with attention you will not suffer from this and your device will have a better resale value, see vendor support article HT208191.
Use this app to visually check your OLED screen if you have signs of "Image Persistence" or "Burn-In". Read about the best way to take care of it.
When you want to buy a second-hand iPhone with an OLED display, you can use this app to visually check for damage to the screen.
This App does not claim to fix or repair an OLED screen which has Image Persistence, or burn-in.
You can also enjoy two demo videos to check out how good looking Dolby Vision and HDR 10 are, on your iPhone OLED display.