My initial reaction to Google's announcement of a Chrome extension allowing users to block sites from appearing in Google's search results was generally favorable. That is, at first blush, I thought "well, at least Google – unlike Blekko – is allowing individual users to determine what appears in their SERPs, rather than relying on the opinions (with a heavy emphasis on opinions) of other users to say what sites should appear in search results, and which should not."
Upon reconsideration, I'm not so sure. Even my initial approbation was dampened by the prospects of what Google might do with that information, given that they "will study the resulting feedback and explore using it as a potential ranking signal for our search results." As Barry Adams said in an article in the Belfast Telegraph, "it will enable internet users to indulge in their own confirmation bias on Google's search results." By extension, this confirmation bias could then be used to fuel what appears in search results algorithmically.
Historically Google has recognized that this is a potential problem, at least insofar as it took action to mitigate the impact of "Google bombing" – the concerted effort by multiple users to elevate a resource to the top of the SERPs by the leveraging of focused anchor text in links. Regardless of whether you think either George W. Bush or Bill Clinton could be considered a "miserable failure," what propelled their respective White House biographies to the top of the search rankings for this term was not the general consensus of the web (as measured by multiple ranking signals, including some basic ones missing here, such as the use of the key phrase in the referenced content), but might be called "advocacy SEO." Is it not possible that similar tactics could be employed by Chrome users to game Google's results?
However, even if Google decides not to use the extension data algorithmically, users who do block sites from their search results will experience a shrinkage in their individual web universe. I think that one of the wonderful things that the web has brought to human discourse is to offer a huge smorgasbord of thoughts, opinions and facts (as, of course, "facts" are not wholly objectively-based) to the information diner. That menu becomes a lot more limited when whole categories of food are excised from it. As Adams so eloquently puts it in his Telegraph article, the "new Google feature allows users to separate themselves from that global conversation, and to only be exposed to ideas they find comforting."
Another thing that strikes me both about the Chrome extension and the Blekko ban is that a "site" is a terribly broad designation. One may encounter a piece of spam on a particular domain, but with these measures one is not marking a piece of poor content, but any and all pieces of content that are published on that domain, regardless of their quality measured against that one resource. In it analogous, in pre-Internet days, of walking into a library and, while browsing the stacks, skipping over any books produced by particular publishers. In that scenario, a reader who struck Penguin from their list of approved vendors because they publish The Communist Manifesto might then never discover The Fountainhead.
And of course, while Google's intention with the Chrome extension is to target "sites with shallow or low-quality content," there's absolutely no guarantee that these will be the sites targeted by users. That is to say, Google will be blind as to why a site was blocked – quite different than the verbose justifications required of users when they submit a spam report using Webmaster Tools. There's a world of difference between dampening rankings or blocking a site because they use sneaky redirects and imposing penalties on a site because a bunch of anonymous users don't like it.
Clearly Google wants to demonstrate that it's "getting tough on spam" but, again, what content may ended up being impacted may not be spam at all. Even by the very blunt measure of headlines designed to rank highly in search, the Huffington Post, as has been recently highlighted, is of guilty as this tactic as eHow (rather ironically, as Penny Herscher's widely-linked article on "spam" in Google was published on HuffPo). And even when a Huffington Post piece is spam, I'd argue that a user who blocks that domain on the basis is depriving themselves of a lot of news, opinion and information that is very much not spam.