"by giving Google hints as to what searchers don't like."

Crowd-sourced curation -- this must be Google's eventual goal for the "blocking" feature. Can't content farms be thought of as spam? And, if so, don't the same techniques for spam identification apply, especially the "mark as spam" button in Gmail?

[Conjecture and speculation below as there's no evidence Google is going to modify search results based on users' blocking propensities, but I think the possibilities are worthy of consideration.]

I wonder how this scheme could be gamed? Consider how one might game spam filters to cause a target's future mailings to end up marked as spam. To hit a high-volume target, one would have to cause a ton of email addresses under his control to be placed on the senders distribution list. Then, when messages arrived, he would mark them all as spam. Although the total percentage of the sender's mailing list comprised of the bad guy's email addresses might be small, this technique would still be effective because the percentage of "activist" recipients required to flag any sender as a spammer is so low (at least I'm told). To address this problem, Google could certainly apply schemes similar to those used to identify click fraud. Such countermeasures would require the bad guy to make his attack appear more organic. Have there been actual instances of such denial-of-service attacks?

With a similar strategy, could, say, an anti-abortion group enlist its membership to banish pro-abortion groups' websites from search results? What if members were instructed to search for phrases like "How to get an abortion" and block Planned Parenthood, etc.? The difference between websites and email lists is that the owner of an email list has direct control and could take countermeasures like changing mail servers, etc. Also, large bulk mailers (MailChimp, etc.) generally have good relationships with large email box providers like Google and can therefore plead their clients' cases directly to a responsible person. How would pro-abortion groups get attention from Google if their site rankings dropped due to such grassroots organizing? Would Google be able to identify this as an orchestrated effort?

How would the behavioral patterns of pro/anti-abortion activists be any different than this forum's treatment of Experts Exchange? Google will observe that one small segment of its userbase blocks the site in high numbers but another segment, when searching for the same terms (MySQL failure XYZ) click on Experts Exchange results? Wouldn't users actually seeking information on how to get an abortion recognize the name "Planned Parenthood" and click on those same results which were blocked by activists?

Perhaps the solution would be personalised weightings for search results. You could then build a weighting graph based on blacklists, e.g. if I have a pro-abortion site on my list and it's found on someone else's blacklist then I would have a negative weight for the other sites on their blacklist. Maintaining the graph and weightings would be massively expensive though so I'm not sure when it would be viable.