I assume it displays varying shades of full-screen grey. There's no public brightness API. You're free to do it outside the app store, of course. (you can find the private API calls pretty easily on Google)

It looks like it is just sending a total blank white screen while it takes a picture. Sure you can change the intensity-- use a gray pixel instead of a white one.

Actually, it briefly displays a sequence of four illumination conditions - a white semicircle at the top, right, bottom, and left edges of the screen.