Improving the contrast of the rear projection screen is achieved in different ways. The key issue is controlling the impact of ambient light hitting the screen.
The simplest way to improve screen contrast is by tinting the screen material a darker colour. But this reduces light from the projector being transmitted through the screen, thus reducing gain. This can only be counteracted by using a more powerful (and expensive!) projector.
More sophisticated techniques use one-way filters which absorb ambient light, while still allowing projected light to be transmitted in the other direction. Thus high gain can be maintained.
One interesting example from the various patented dnp technologies is the dnp UCS Screen. The front-facing element has a microscopic lenticular layer with a highly profiled reflective ‘light trap’ that dissipates ambient light by cross reflection rather than reflect it back to the viewers.