If the screen turns black when using Voice Band, just move your fingers away from the proximity sensor. The iPhone has a sensor (located above the screen to the left) that senses when you put the phone to your face (like when you're on a phone call). Voice Band uses this to lock out the screen so you don't accidentally press buttons with your face.

So, if you're holding the phone in front of you, just try to keep your fingers away from the sensor.

If you use headphones, there's no need to do anything, as the sensor is disabled.

Thanks,
Rim