In P6 with a Poser box prop scaled to 10%, then imported as an obj, I found that if I had 'Auto Settings' selected in the Render settings, the box rendered black, but if I had 'Manual Settings' selected, the box rendered correctly. I have no idea what significance this may have, but thought it worth passing on. This may be why some people don't get the problem, they may be using 'Manual Settings'.