Screen Blackout Box

[Desdichado]

I'm trying to find a device that allows me to switch a monitor to a black screen with the push of a button. The idea is to plug a vga cable from the computer into one side, plug the monitor into the other side, and use the box to switch between displaying a black screen and displaying whatever from the computer.
I know such products exist, because I've seen them used, but I can't seem to find one anywhere. Any help here would be appreciated.