Blankmon is a tool to blank monitors. I mainly use it while watching movies, so my secondary monitors do not distract me. It simply displays a black fullscreen window on each specified monitor and tries to place itself above all windows.
Start the exe and pass the monitors you want to blank as parameters. Example: blankmon.exe 2 4 This would blank monitors 2 and 4. A second start disables the blanking (regardless of parameters).