When did we decide that off-black backgrounds were cool?
It seems like the number of themes with a truly black background are extremely rare. I like the contrast of a true black background, but most "dark" themes seem to have a grey or slightly blue tinted background.
I'm really curious, how did we, collectively, seem to decide that a dark theme should have a off-black background? Why do people like it over a true black background?
It boils down to readability (not only text, but all controls also). While in theory high contrast is desirable, in practice contrast can be too high, and reading pure white text on pure black background is harder than reading dirty-white text on dark blue background - too high contrast kind of blinds the eyes.
One other, less important reason: if you want to have a drop shadow, then your background had better not already be truly black.
Because pure black doesn't exist in nature and usually is perceived as jarring. Common design principles therefore advocate for using a colour that's almost - but not actually - black.