This may be an issue with the exposure settings for the slow motion camera. Ensure that the top knob on the camera itself is all the way to the right. Then go into the Camera Settings tab of PBU Settings and select the slow motion camera.
If the exposure is set all the way to zero, it will display a black preview screen. Change the exposure to a higher value to solve the problem.
Also, please note that the only slow motion camera compatible with PBU is the Point Grey Flea 3.