Hello @Chungulis, welcome to the forum! 
I love your avatar!
Sorry to hear about that! It sounds like something is blocking the proximity sensor. In your phone, you have a small sensor that makes sure that your screen isn't active while you are holding your phone against your ear. You can locate the sensor with help of this overview, the sensor is marked with the number 4. https://support.sonymobile.com/global-en/xperiaz3plus/userguide/overview---iy/
Make sure that the sensor is free from any screen protector or dirt, and wipe it clean with a soft cloth. You can also test the sensor in Diagnostics, a super smart app you will find in Settings > About Phone > Diagnostics/Support (depending on your Android version). Run the test on the proximity sensor.
You should also put your phone in safe mode, to make sure that no third party application is interacting with your sensor. In Safe Mode, make a call and see if your screen still turns black. If it doesn't, you know that it is a third party application that interacts with the sensor, and you should take a look at your downloaded applications.
If you have an system update waiting, you might want to update your phone. You can check if there is any waiting updates by going to Settings > About Phone > Software update.
Let me know how it went, and have a nice day!
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