Check Tools>Options>Theater View>Behavior>"Fade out after inactivity to prevent burn in". I think that it is "checked" in your system and is causing the black screen.
EDIT: No that's not it. My Theater View screen was also black/dimmed-out this morning when I turned on my TV and switched to the HTPC input (I don't shut off or sleep the HTPC), and I have the box I mentioned above unchecked. So it's something else.
But the Theater View screen does come up to full color/brightness once it sees activity.