I am not a dev but is there a way to know when one of the two screen buttons is being pressed? If so, and it'd have to be almost immediate, but you could fuzz the image or make it go black so the screenshot is black. Is this possible?
Contrary to some other platforms (to my knowledge, most notably Android), iOS apps are very sandboxed.
Edit: I have personally confirmed this on iOS. According to a commenter below, this is also true on Android.