How To Turn Off Mac’s Monitor screen
You didn't describe what kind of Mac it is, but I'll assume an iMac or MacBook of some flavor. If you had a Mac Mini or a Pro with a third party monitor then you probably wouldn't be asking this question.

You can't do exactly what you want with an iMac, but if you use "Apple Remote Desktop" (App Store, $80) you can lock the screen so anybody sitting at your computer can't see what you are doing. This doesn't seem to be possible with the built in screen sharing.

The Apple Remote Desktop option is more secure, since you can't be sure that nobody turned your PC monitor on after you leave work.