Here is a simple and funny keyboard trick which you can use to impress your friends. This is nothing but less known feature of the Windows. You have to basically turn on high constrant. Here are steps:
1) Hold Shift, Alt and Print Screen keys together.
2) Click on OK when a message appears (this enables high contrast on Windows system).
Now your screen should change to black.
3) To revert back screen to previous state press Shift, Alt and Print Screen again.