Let me be clearer: Being able to show black is super important for AR. For one thing, it's a pain in the ass to read text on a transparent HMD , because you never know what colors will be behind the letters. You can make some educated guesses, and put up your own background colors for the text, but since everything has opacity, it'll always matter what's physically behind the "hologram".

Yes, yes, it's still not a hologram, but the popularized version of holograms (from things like star wars) is still the best way to think about AR display.

If you can show SOME black, text legibility becomes a lot easier. Everything can look way better, even if the world always shines through enough to see it.

If you can show PURE black, enough to ACTUALLY obscure the world, now you can erase stuff. Like indicator lights, panopticon optics, and people.