Hi JaviC and welcome to the Xperia Support Forum! 
I am very sorry to hear about this.
At the very top of the display of all Xperia phones there is a proximity sensor intended to detect whether you have the phone to your ear or not. When holding the phone to your ear the screen is meant to go black in order to prevent you from taping any functions by mistake with your cheek. On the Xperia X the you can find the sensor right next to the speaker (see point 6 in the overview).
Do you have a display protector mounted on your on your phone,or any cover which could cover the sensor? Try removing it in that case to see if the screen still goes black during calls.
You can also try testing the functionality of the sensor in the support application. To do this, find and tap Settings > Support > Tests > Proximity sensor.
Try this and let me know how it goes!
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