flux had an iOS app, but it was always side-loading through Xcode (or for jailbroken) but never on the actual iOS App Store. They need to use private APIs so it was never authorized in the first place.
If I recall correctly, they started with doing a screen overlay, and migrated to using the hardware gamma tables (I think I might have pointed them to it?). Android apps that do screen dimming use overlays, which ends up causing flickering.
