Sorry for the shity quality, but I used ipad 2 to record this video. It might be hard to notice but pay attention to the corners of the display.
As I rotate the phone, the background readjust with slight delay, revealing black screen color underneath the gradient background.
Look at the area around home button.
As for the illumination. No, there is no back light. The pixels are illuminated individual one by one. And as I said above, the gradient is often used to eliminated the bleed and ghosting while scrolling white text, and to add depths.
Did you try to make a graphic on computer, that is 100% black and view it in gallery ?
It shouldn't display as gradient.