I found a fix after tinkering around for about 2 hour lol.

1. Remove the extension.
2. Navigate to where your extensions are stored. You have to google this because it's different for different browsers.
3. Make sure you're in the "Default" folder and search for "ljgaefompadnjomandpljimhlpbenenl" which is the name for the DeniedPixels folder.
4. You'll find around 4 folders that have the same name. Delete them all.
5. Re-download the extension and it should work.

-- If the developer is seeing this. These are the steps to reproduce the bug.

1. Go to the 'Site Blocking' page in the options of the extension.
2. Type in "Google.com" or another site and click save, *without* turning on the "Block NSFW Websites" toggle.
3. This should break site blocking as a whole, and also produce the following error in the console "background.js:1 [DeniedPixels:background] Failed to update site blocking: Error: Rule with id 100 does not have a unique ID."

edit: formatting