The technology involves a pair of filters which help to reduce the amount of ambient light that the screen reflects. The filters polarize the light, changing its direction before it reflects off the screen behind. When the light travels back it is unable to pass through the filter and is blocked. We noted in our Lumia 800 review that "you'll struggle to distinguish where the display panel ends" and it's by preventing the ambient light from escaping the screen that Nokia achieves that effect. The light produced from the display itself, having not passed through a polarizing filter, is able to escape without being affected. You can find a step-by-step breakdown of the process over on the Nokia Conversations site.
Nokia's ClearBlack Display technology explained
Nokia's ClearBlack Display technology explained/
Nokia has produced a graphic explaining how ClearBlack technology works