Can I blacklist a website from 1Password?

Do you mean preventing 1PW from offering to autosave an entry?

With 1PW7 and earlier this was in the settings, under 'browsers'. You would fill in your chosen domains to ignore there. There had also been an option on the popup to not ask again, which would similarly populate that list automatically.

I didn't see any similar functionality in 1PW8 unfortunately, either in the standalone desktop app or in the standalone browser app. I had tried to find this without success, but someone else may know where to find it now.