Its not quite "Instant", but it will do it in 1 minute.
Open script editor and paste this in:
do shell script ("pmset dim 1") password "YourPassword" with administrator privileges
delay 100
do shell script ("pmset dim 30") password "YourPassword" with administrator privileges
Replace "YourPassword" with, well, your password. Save it as an application and drop it in your dock for one click access.
What this will do is set your display to sleep in "1" minute, wait 100 seconds, and then set it to sleep in 30 minutes, your display will already be asleep, if you didn't touch anything. You can of course modify the times to your taste.