The blacken PDF Tool renders modified pages to an image and embeds that image back into the PDF. The quality decreases by this step but the page should still be readable. Can you send me a screenshot or the PDF files to firstname.lastname@example.org so that I can have a look?
I have the same issue. Blacken PDF Tool decreases quality significantly.
Although it's still easily readable for me, older people have problems with reading the content of printed documents. Fonts gets a little too blurry for them.
Having an option to increase quality even at the cost of file size would be great.
There is a setting option with the name "blackenSecurity.dpi". Here you can set the DPI value that is used for blackening. The default value is 150. This value can be created as a DWORD in the registry path HKLM\SOFTWARE\PDF24 or HKCU\SOFTWARE\PDF24.