I had a similar problem with my wifes wildfire. She made the call but once lifted to her ear it was black screen and even when lifted away it was black unless shaken.
The fix was to recalibrate the G Sensor.
You will find this in Settings / Display.
This requires the phone to be placed on a true flat surface with no incline.
2 presses of the button and its done.
Now your phone will be more accurate, knowing if the tilt is away from your ear or not. So illuminating the screen or not saving power and pressing buttons by mistake.