Why my display Nexus 6 (Android 5.1) became black when I make or receive a call?

[AC Question]

In my Nexus 6 the display became immediatelly black if I make or receive a call

 

[Rukbat]

So that when you put the phone to your face, you don't dial with your cheek. If should come back within a second after you take the phone away from your face. If it doesn't, either you have the proximity detector blocked by a screen protector (so it registers a "face" near it at all times, or the sensor is defective.