the idea of a screen shade isn't one I've thought about before, but I can see how it could be useful! I was curious whether there is anything out there in the freeware community for such tasks.
I've found this tiny download and think it works pretty well (I asked my dad, who is sighted, to tell me if he could see anything on the screen). It's a program called Dimmer, made by a guy called Nelson Pires. The website says “Dimmer is a little free application designed to help control the brightness of your computer.”
You download it and extract the zip file into a folder. There is an exe file that runs the tool; there is an xml file that saves the settings, and a small help file. There's no installation, it just runs and to uninstall it you just delete the folder.
When you run it, it places an icon on the system tray. That took me a while to figure out how to access it! In Win10. I used Cortana to go to taskbar settings where there is a list of all icons that are running on the task bar and system tray, along with check boxes to say whether the icon should be visable or not. Most are selected to not be visible. I used the tab key to work my way down the list and then switched to object navigation to activate the show setting for Dimmer.
When you find Dimmer on the system tray, use the application key (between right ALT and right Ctrl) menu and find the configure... option. Press enter key on that and you get a window with an opacity slider. You can choose between 95 percent opacity (almost nothing viewable on screen) and 5 percent opacity (almost maximum brightness). You can use home and end buttons to jump to 5 or 95, or the left and right cursors to choose what level you prefer. OK, it's not a single hotkey but it'll probably be a stop gap until anybody wants to write an addin to achieve 100 percent opacity.
For comparisson, there is a very similar freeware called PangoBright but that apparently only reduces the brightness to 20 percent / 80 percent opacity and finding its download sites drove me mad! What did amuse me was that the reason Dimmer is limited at 95 percent opacity is because, for the sighted folk in the world, they wouldn't be able to turn the screen on again when they couldn't see the window ... #BlindIsBest
Here's the download page for Dimmer 1.0 on the author's website: http://www.nelsonpires.com/software/dimmer/
Hope this is of some interest :)