If the screen of your Android phone goes black during phone calls but doesn't come on, thereby, not allowing you end calls since the power, menu and back button won't work then your phone's proximity settings has been tampered with by some app or a screen guard.
* A temporary solution / trick around this is to Go to Settings > Accessibility > Tick 'Power button ends calls'. This enables you end calls using the power button in such a scenario.
* A permanent fix is to uninstall the faulty app or a factory reset
* If the phone has a screen protector, guard or pouch at the screen's side then consider removing it as it's likely blocking the proximity sensor