A black screen can help you identify any dust particles and clean them. It can also be used to turn one screen dark while you keep working on the other. Another important use of the black screen is conserving energy because a black screen turns the darker pixels off on modern OLED displays. If you have a stuck pixel on a screen in a different color, a black screen can help identify it. Unfortunately, you cannot identify a dead pixel with a black screen because dead pixels are usually dark/black already.