I see various solutions when I search.
Could be the proximity sensor is obstructed. See KB27934-BlackBerry smartphone touch screen goes dark during a call due to proximity sensor
Upgrading the OS may cure the problem.
Turning on the speakerphone ($ key) and then turning it off is reported to turn the screen back on, which would be a work around.
I'm still using the OS that came installed and have not experienced this personally. I tested and the display goes off when i cover the sensor and comes back on when I uncover the sensor.
I would try a hard reset first. Then, if you can't correct it with an OS upgrade or an OS reinstallation, my advice is a warranty exchange if it is under warranty.