So it goes black when it's up against your ear, right? Does it come back when you take the phone away from your ear? That would be normal function for the proximity sensor. But if it remains black after taking it away from your ear (or after the call is done), then it sounds like your proximity sensor is having problems. Screen protectors and cases can sometimes interfere if they are blocking the sensor (which is near the earpiece)--e.g., a dirty screen protector, or a case that hangs over the sensor.