[kjkjadksj]

I don’t know if that would do it or just return your dark current image.

The best thing to do is just take a file same as your screen resolution into your favorite image editor and fill it with actual true black. Save as png and send to phone.

[myfonj]

Was not sure if sensor in pitch dark would catch some other radiation and output some pixel "grain" seen from high ISO or not, but from what I tried, visually the photo seems pretty black, so the method seems to be surprisingly usable.

As for "same image as your screen resolution": screenshot sounds like the exact fitting thing here. As a challenge, tried making screenshot black using stock Samsung "Gallery" and it seems that repeated Edit - Brightness: -100 - Save as copy, then open the copy and goto back to Edit can do the trick as well, after four or so copies. (Copies, because there is no way to re-apply same effect on the same photo, apparently.)