As much as I'd like to be able to claim to be a color expert that can eyeball HSL values with enough certainty to give the answer in decimal percentages the above numbers are from color picker readings of a screenshot of my install.
> I'm able to find plenty of people complaining about the dark background being pitch black[0]
I can't speak about what was true 4 years ago in a preview build adding dark theme, or Windows 10 at all for that matter as I don't have a current install anymore, but for Windows 11 today the above are the measured values.
> That's only for surfaces with blur enabled though, like the sidebar in Settings or the taskbar. Non-blurred surfaces are pitch black.
For that particular preview version of 10 perhaps, in Windows 11 there is also a new opaque material called Mica used heavily throughout the interface as it has less of a performance hit (no blur/transparency) https://docs.microsoft.com/en-us/windows/apps/design/style/m...