The Dark Zone is a large area of the Milky Way (from about 25° to 55° longitude) that is obscured by a complex of dust clouds called the Rift, so-called because they appear to create a large tear or rift in the galaxy. The Rift clouds obscure important parts of Vulpecula and Cygnus and blot out most of the Dark Zone. Fortunately, astronomers can see through this dark dust by detecting other parts of the electromagnetic spectrum such as radio and infrared. And there is plenty to detect - the Dark Zone includes major star formation regions such as W42, W43, W47, W49 and W51. A schematic for the overall structure of the region near W43 can be found in Figure 9c of this 1996 paper.
I've split the Dark Zone into two maps. The first map is an overview and includes a few outlying objects. The second map includes the objects closer to the Dark Zone central region.