If the Windows Magnifier (press Win and + keys) is displaying a black lens or screen window, then the likely cause is the NVIDIA graphics driver. The fix is to force the magnify.exe proggie to work with the standard GPU - not the NVIDIA GPU.
(FIX copied below - my numbering of the solution steps.)

Windows 10 Magnifier Lens Window is Black

By BGGRIT on 2015-08-13
I have set up my magnifier (size, shape, magnification, and to auto start), but the lens window will randomly turn black (not the entire screen just the lens window).  Makes it difficult to navigate with my vision issues.

Any solutions would be appreciated.  Thanks
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...further down the page, it says:

tonycol Replied on January 16, 2018
Reply
In reply to Major Instigator's post on January 15, 2018

I am by no means an expert on Windows 10, but I can let you know how I resolved the problem on my own PC. My PC has an nVidia graphics card which works just fine with all applications with the exception of Magnifier.

• Ensure nVidia Control Panel is installed and enter the control panel via a right click on the desktop screen, then select the nVidia Control Panel option in the context menu.
• On the left side of the application which is open, there is the option for manage 3D settings. Select this option. I know you probably aren't bothered by 3D anything - it's just where the settings are.
• On the right side of the screen select the 'Program Settings' tab.
• If the drop down selector in part 1 does not contain magnifier (and it probably won't,) click the 'Add' button and then the 'Browse' button.
• Navigate to c:\windows\system32\magnify.exe and double click on it. Doing this will add it to list.
• Choose c:\windows\system32\magnify.exe from the list.
• In part 2 choose integrated graphics from the drop down.
• Click the Apply button.

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@science2002:

Just for curiosity, as a notetaker, are you now using OneNote or what? If so, will you stay with ON even when (from v.2019) it will save only to the cloud?

-science2002 (October 12, 2018, 03:19 PM)

I shall probably stick with OneNote 2016 for the meantime, but am experimenting with a possible migration strategy --> IQ (InfoQube). Trouble is, ON is just so good at coping with the various data types that I use. Hard to beat.


@superboyac:

... My life has taken some drastic turns the last 5 years.  I hope it's for the best, and it's not something I look forward to repeating. ...

-superboyac (October 12, 2018, 03:57 PM)

Yeah. I guess I know what that might feel like:

Life is what happens to us while we are making other plans.
 - Readers Digest, January 1957.
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Hang in there mate.