I'm not sure how this would be implemented. Where would the blacklist be held and how would it influence the search results? I know that they already do a lot of search customization but most of it is just aggregate statistical computations. It's not that they return results specifically tailored to you but more like results tailored to a very fuzzy average version of you. A blacklist seems way too specific to each user to be susceptible to meaningful aggregate statistical operations like spam filtering which is one of the reasons that spam filtering in google is so good. Each user contributes something and everyone benefits. I don't see that happening with blacklists. I think to make it worthwhile they would need to figure out how to feed the information from blacklists into providing more meaningful results for everyone.