I blame what I'll call the Art Teacher's Fallacy: "Pure black does not exist in nature, therefore you must never use pure black." This is equivocating on the meaning of "pure black"; your paints or computer screen also exist in nature and are therefore not "pure black" in the former sense.
In print, people went to great lengths to get the highest contrast possible. I see no reason why we shouldn't do the same on screen.
I did an early project with only #ffffff, #000000, #ff0000, etc because I thought those were the most “pure” colors. It looked freaking awful.
The best colors are ones that are subtly off of pure white and pure black; to the point where you can’t even tell. My website uses #191919, which you can’t consciously tell isn’t black, but looks better for it.
Print’s a different beast. I used the same concepts to design my business cards and they looked washed out using off-black. Paper reflects environmental light (instead of emitting its own light) so you can crack the contrast as high as you want and the end result won’t be pure black on pure white.
LCDs imperfectly block the backlight to make obviously impure black for #000000. Every OLED I've seen has slight light emission for #000000 (leakage current?) too. And very few monitors are used in a perfectly dark environment, so there's light reflecting off the screen even in the #000000 areas.
