Video projectors have limited sequential contrast. This means that, in dark scenes, background leakage light is always visible.
When multiple projectors are overlapped and electronically blended to form a larger image, there will be unwanted double-intensity background leakage. This becomes most apparent when dark-scenes are projected.
The image is an exaggerated example, showing how ‘leakage’ black levels cause unwanted bright-ups in the overlap zones, and the importance of optical blending.