That sounds pretty foolproof so long as your black box fill method doesn't fill with a 99% opacity, or a flood fill leaves behind a few invisible anti-aliased pixels, or the merge operation of the black box doesn't result in some multiplication leaving a few bits of difference. Even if you erased the layer below, then filled above, I've had erasures vary in the bits outside the alpha channel messing up games using the texture info.
Overall, I kind of understand the paranoia even though in principle it does sound pretty foolproof.
Wouldn’t all those fears apply to printing as well?
The alpha channel ones would not apply to printing, and overall printing is an extremely lossy operation, where all those minute details get washed out in approximate ink levels and the muddiness of the physical world.
It might not be totally foolproof, esp for a very accurate print process (don't use your photo printer maybe), but it's probably many orders of magnitude noisier..
I think if you're really concerned, you'd print it once, apply physical black tape on it (or cut out with a razor), then scan that :)
If it does, then Export to PNG almost certainly removes it (while also removing all other selectable text)