"Darkness!" zones extinguish any and all light sources. These include lamps, torches, The Seeker's Ballad, the Watchwood Melody, and any magic light spell. The message "Darkness!" appears on the text window.
Using a light source while within such a zone causes a very brief flash of light, possibly helping to get bearings when exploring.
When combined with other hazards, darkness zones have the potential to be very dangerous.