The device screen will go black during a call if anything covers within 5cm of the proximity sensor. Usually this sensor is close to the front camera. If you make a test call you can see if you cover the sensor screen goes black and of not covered will come on again. But could also be a case of something covering it. If it's allways black during calls. This feature cannot be switched off as it saves power and prevents phone from overheating during calls. Attached photo example location.
