Why doesn't Google block sites manually (sites that are obviously bad)?
Off-topic, but I've always thought this is the main reason Google Blog Search isn't very good. In the quest to make an all-algorithm blog search, Google sacrificed having one at all. If they had simple restricted the search to blogspot.com, wordpress.com, and typepad.com, and applied the normal spam filters, it could have been pretty good!
This does not mean that every kind of discrimination is equally good. Dunking a site by an algorithm (sensible disclaimers apply) looks much better in court than a decision made by a human. The latter is vulnerable to questioning based on (supposed) hidden motives.