Why is my screen going black at night when I make a call

You seem to be having some sort of interaction between your ambient light sensor and the proximity sensor.

The ambient light sensor tells the phone how bright the room is in order to adjust the screen brightness under different conditions. In a dark room it dims the display so you're not blinded, and in daylight it brightens the screen so you can see it against the light.

The proximity sensor is primarily used to detect when your face is next to the screen (as when you're making a phone call) or the phone is laid face down on a surface. In those cases it turns off the screen, assuming it's not being viewed under those circumstances.

Just to confirm, the display does go black when you make a call during the day and turns back on normally after the call is complete, is that right? As to why the proximity sensor is failing when you're in the dark but working in the daytime, I can't even guess. Does it go black when you dial the call, or when you put the phone next to your face?

If you cover the sensor with a finger, the screen should go dark and brighten back up when you remove your finger. Likewise, covering the ambient light sensor should dim the screen and holding the phone under a light should cause it to brighten. Sorry to say, I can't tell you where on the screen those sensor are, but they're normally near the top of the screen close to the front camera on most phones.

Unfortunately, information on the V81 model seems to be difficult to come by; I couldn't even locate a user manual for it, let alone repair guides or information on your sensors. Hopefully some of our repair techs on this site will have better luck.

There's always a chance a screen protector or a case is interfering with your sensors, so be sure to try removing both of those and seeing if the behavior changes.