Blekko is certainly interesting, specially with /seo slash tags. I think there is space for blekko where Google does not return interesting result. After Google instant its becoming place where you should find something in first 1-5 seconds. If not, its lot harder to find on Google. Blekko on the other hand you can run queries with different dimensions running slash tags (eg. you can sort by date with \date if you want liberal perspective you can do \liberal etc). So Blekko to me is becoming more of a search tool. Its certainly a niche, but i think there is space for these niche search engines. Another good example of niche search space is wolfram alpha.