These 2 files need to be added after a new installation of nightwatch-visual-testing
https://www.npmjs.com/package/nightwatch-visual-testing?activeTab=readme
They should be placed adjacent to the regular files with similar names, but without the "NotBlank" addition.
File locations:
node_modules\nightwatch-visual-testing\assertions\compareScreenshot.js
node_modules\nightwatch-visual-testing\utils\image-diffs.js

To test the ability to test a blank page, these websites can be used:
http://www.blankwebsite.com/
https://blankwhitescreen.com/
https://blank.page

To generate the initial blank baseline image, you can use this URL: https://blank1.amonthlybudget.com/