Does your screen go black only when you put your phone near your ear (or put the screen near any flat surface)? If your screen goes black even if you don't place it near your head, i.e. immediately after a call is answered, make sure that nothing is obstructing the proximity sensor of your phone. That can usually be found near the earpiece. Make sure that your phone case (if any) is not covering the top part of the screen, and that there are no excessive traces of finger prints, dried up sweat, or any other foreign debris near the earpiece.
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