Video black: not real black, visible, and annoying, the curse of projections. The good news is since this problem is fairly global, there are alot of solutions. At UMKC, the typical CD-Rom shutter is a typical solution (it was also demonstrated in the 2007 Tech Expo), but we have found that it can be temperamental at certain angles. Rope, pulleys and a piece of cardboard have also produced usable shutters as well. We have even used pneumatically controlled shutters (with the dowser on a piston) which worked well (the noise was minimal since an exhaust was used.) However, if you use projectors enough, and fine control is important, there are a number of DMX controlled shutters now available commercially.
400 including shipping. Very lightweight (under a pound). pretty simple system with a shutter that covers the projector.
Slightly more expensive than the above product.
from City Theatrical, much more expensive (775 list)