According to the site on Firefox Add-ons [1], it tells you which websites it is not showing, which I think would not be possible with your suggestion.
Although - like zargon says in a top-level comment - this clearly should just be built into search engines themselves. The only semi-legitimate reason I can think of that they wouldn't be is that results for a given query would be even less reproducible than they already are from person-to-person. But since Google already "personalizes" results, I don't think that's really a factor for them. They could just have a little button at the bottom that says "3 results from sites you've blacklisted - wanna see them?" or whatever.
Google News does this on every link (Hide stories from XXX, More stories like this, Fewer stories like this) so clearly the feature exists.