The proximity sensor is turned on and teh screen goes black when you hold the device close to your ear in order to avoid misoperating the app.
You cannot disable the feature during a call.
If you want to keep the screen lit up during a call, check where the snsor is on your device and be careful not to cover the sensor with your face or hand.
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: App version 1.7.15 or later for iOS supports the above.