If you hold your device close to your ear during a call, the proximity sensor enables and the screen turns black ( in order to prevent you from clicking the calling screen mistakenly.) If you put the device away from your ear, the screen lights again.

If the screen turns black even if your hand or face is not close to the device, please check whether the phone case or the protection film covers a part of or all of the sensor.

In iOS, the proximity sensor turns disabled while you are talking with the speaker ON or making a video call, and the screen doesn't turn black.
Note: The operations might be different depending on type of device, OS or version.