Well that was a half assed response.
What you can do is , if you have Cydia, you can get a dimmer that completely turns off the screen to black ( set the backlight to fade to black after 5-10 seconds of inactivity, so you can at least have a few seconds to set up the video how you like it...and absolutely make sure you have the "autolock" set to never...you dont want to be literally tapping the screen in the dark...maybe this is why Itunes might not have a total blackened dimmer app).
Not sure if there is an Itunes app that has a dimmer that sets it to black, but you can at the very least dim the light to the lowest setting to at least save as much battery life.