With a projector, you can't "throw" nothing (aka black). As a result "projected black" is simply lack of projection.
In the case of a translucent or transparent reflection or waveguide surface - which is what the projection reflects off of - "black" is whatever the darkest part of the surface is. In effect whatever else is emitting from the surface that you're looking at will change the depth of "black" you get.
This is why the Hololens and other see through AR devices are always tinted, to set a higher threshold for "black" than the surrounding unaided view.
The are three layers of polarizing material. The two outer layers are at right-angle polarizations to each other and normally would be completely opaque on their own. When power is applied to the liquid crystal, it twists the crystal's polarization to be at a 45° angle to the other two layers, which then permits some of the incident light to pass through.
An optically transparent waveguide display can use an LCD layer to block light coming through the front and then not render graphics on that area of the display. It will be opaque black at that point (though rather fuzzy around the edges, as the LCD won't be in focus).
Magic Leap 2 actually employs this technique. It's... a lot like the rest of the device: a good idea on paper.
If you forgo the projector part and replace the mirror with a transparent LCD, it’s just too close to your eyes and you can’t see anything. If you add a microlens array to the LCD, now the LCD might come to focus but background becomes way too far-focused, and you can’t see anything either.
If we were on an Enterprise-D, I guess I could just ask replicator for a passive illuminated metamaterial light field image combiner with integrated processing than runs on bus power from DisplayPort input, but we are not there yet.
So, for now, our AR HMDs can only brighten pixels against backgrounds.
