Think about it: When I point a projector at a white screen, and display an image with black regions, those regions are actually the same color as the white screen they're projected onto. "Black" is entirely defined by its contrast by the surrounding regions, so just having less-bright areas will give you at least an approximation of black.
I think that would defeat the whole purpose of the display being transparent so that you can see your hands. The display itself is not "3d" but is simply changing the perspective displayed based on the position of the user's head.
Adding an LCD layer to a transparent OLED screen just gives you an alpha channel, effectively. Independent control over emission and transmission. I've thought about this many times when my mind has occasion to wander.