I wonder why no one suggested putting a very thin, transparent monochrome LCD behind one of these. It would allow darkening the background, as opposed to OLED's ability to emit light but not block it from passing through. Basically, you'd use two display technologies to complement each other.
Transparent screens could be useful in all sorts of scenarios, but I think tech demos too often ignore the fact that, most of the time, you want the image to be as clear as possible, which means opaque.