Proximity sensors are located at the top of the screen section. Stuck on black usually indicates that the sensor can't see through the lens in the screen.
This is remedied by either using like a purple marker to let the sensor see through properly or adjusting the distance of the sensor to the screen. Closer is better. If marker doesn't work use a q-tip with a bit of alcohol to clean it off the surface.
Any plastic temporary covers on top of the screen and dust will cause the sensor to not work properly.