The lack of this feature on Google shows whose interests they truly have at heart.
Or they assume they know better what the users wants than the users themselves. YouTube straight up ignores dislikes - and will show disliked video in mixes or at first place of search results.
If we give Google the benefit of doubt, maybe people just kept accidently blocking useful sites and later complained they couldn't find anything? Thought Steam deals with that perfectly: search results start with disclaimer: "8,875 results match your search. 2,053 titles have been excluded based on your preferences." - and it takes 2 clicks to go back to unfiltered results.
[0] - well, at least Gnome project follows that idea, killing any discoverability