Are these client-side only extensions that are manipulating the DOM, or are they somehow feeding parameters to google to exclude domains (by e.g., automatically suffixing a series of "-site:blah.com"s to the query)?
I don't know if I'm secretly being A/B tested, but Google seems to ignore - and "" terms in my queries from time to time.
I have also noticed that and it has been really difficult to search for obscure error messages now.
They manipulate the search result DOM. uBlacklist is the one I use and it'll manipulate the search result page telling you when something has been blocked, allows you to unlbock that site, etc.