Can you use the Windows + D shortcut for one monitor only?

It is possible for free. Swap Screen is very simple program to do that. You have to define other shortcut than win+d, but I still find it convinient enough. For example I bound alt+1 to show desktop 1, alt+2 to show desktop 2.

Additionally, this program have some other neat features. One that I find myself using daily is function to move windows to another monitor with a shortcut, then there are sticky cursor and plenty other features. Swap Screen is a part of DualMonitorTools, free set of tools for multiple monitors that can provide even more functionality.

Here is the screen:

If you have UltraMon you can easily do this with a Hotkey. In the Options menu simply go to the Hotkey tab and scroll down to the option Show the desktop on monitor with mouse and assign any key combo that you'd like.