The Core

You can read about it here:
http://searchengineland.com/blekko-blocks-more-spam-domains-1-1-million-of-them-67668

Of course nobody relies on Blekko for traffic, but every time they do this it embarrasses Google into also tightening their efforts against spam.  Frankly, IMHO, this is not about spam at all: this is about hitting Google where they are vulnerable, that is hitting Google in made for Adsense sites and chipping away at Google's revenue source.