Erm. Well, I do that too, only I use Photoshop. And the printscreen image sometimes turns out black, like what you said. Then I toggle back to WMP, and the screen's black too, until I resume playing the video. To make sure that I screencap the scenes correctly, I see to it that the image doesn't black out in the player when I toggle between windows - I only screencap and then paste to Photoshop when the image stays after I'd already switched to other windows.