Even a small dot in regards of its size called a dead pixel and when you don't take good care of your screen it may become bigger, this is a big factor to consider when buying a new device. make sure to test the device screen by increasing the screen brightness and look for dot area.
Dead Pixel in Android and IOS device commonly occur via factory defect and by accidentally dropping it. the screen suffered an impact and the results is a a dead pixel.
There are also an apps able to download in Google Play Store to test and view if the phone has a dead pixel for you to review if needs a replacement and do a second options.
Also, dead pixel can be fix using replacement. a scenario that a small dot became big and all the screen affected and the results, you won't view details in your screen.
If ever you encounter this, you may go to the nearest technical center and fix it.
Also, Blinking on and off screen can be considered screen and LCD problem and the only thing to fix is replacement.