Q: Aren't there already ways to do this?
A: Not really... Here are some old ways to blank the LCD projector...
Use the function keys to turn off the output - This causes the projector to go to a "blue screen" or display the projector manufacturer's logo. Video glitches also appear on the projector screen. This reflects poorly on you and is disruptive to your presentation
Use a "crash" switcher - A cheap VGA switch that cuts the signal giving you the same blue screen and/or manufacturer's logo as #1
Run to the projector and hit the mute button - Very few projectors have this, and you have to walk over to the projector, find the button, back to your laptop to change presentations, then back to the projector to un-mute it. A lot of walking, and it puts a large gap in your presentation where you can lose your audience.
Use a powerpoint remote "blank" button - Won't work either, because it blanks it on the laptop screen too!, so you can't bring up your next presentation
Putting a piece of cardboard in front of the projector - This makes you look very terrible! Plus you have to do the 50 yard dash back and forth
The VGA Blackout Box truly is an innovative solution to an all too common problem. You could also use a $7500 high end switcher...but this solves it for a whole lot less.