I've got just enough imagination to convert the black to void so it's a very helpful picture, thanks!

What a strange image! I fully expected the black painted portions to just be ... completely black, since after all the whole point is that it reflects nearly zero light. But that's seemingly not the case at all.

You can download the image and put a black stripe over the top of it to compare, and the difference is quite striking. My color picker tells me that the black in the center of the mirror has a lightness of about 17 out of 100 (that's L in the LCH cylindrical color space), which is really just a medium gray all things considered.

I wonder if this is due to imperfections in the paint and surface, or if the camera used just has an extremely high black level for some reason.

Sure it would. And capturing the contrast between the regular wall and the black would give you much better results that turning your monitor off too.

There are lots of cool photos out there of super black material.

I turned my monitor off and saw the most disgusting creature staring back at me. No idea why you'd want to paint that around your house.

If the camera has sufficient dynamic range, then it can give a sense of the effect, even if it's not as cool as the real thing. I thought I remembered reading that small sensors are worse at dynamic range than other cameras though, so a phone picture is least likely to capture it compared to other devices.