Why am I only seeing black when playing on TV or a second monitor?
Standard playback uses an Overlay surface to display video. Some display adapters do not support Overlay on a second monitor or TV-Out port. Some don't have the multi-monitor configuration set up to support it.
Basically you have two options. The first is to set the TV or second monitor to Mirror the current desktop instead of extending it. This will make both displays display the same image and most likely the video will show. The second option is to enable VMR9 Mode (Advanced Option / Playback / Video). VMR9 mode uses your 3D hardware to display the image instead of the overlay surface. It may even improve image quality.