Black screen during Whastapp call/audio
Searching all over the web tonight 1/22/21 looking for an answer on how to fix this and found NOTHING. So with trial and error and a little ingenuity I developed my own solution which worked.

Turn on airplane mode while the black error screen for in call is opened up on your phone.
After the first beep notifying you the the call is losing connection the screen/app realizes it's error and reverts back to the normal state. In call screen will now be normal.
Turn off airplane mode immediately after the in call screen is remedied so you do not lose your call.
You're welcome