This is a good post by Library Clips. Read more here: Bloglines blog search engine
The relevancy is based on subscriber numbers (so anything you write may be relevant to the search term by default, as you have lots of subscribers)…Technorati bases it on incoming links (again what about the long tail, and also this is just popularity)…whereas Sphere bases it on a number of things (incoming links, subscribers, content analysis, comments, etc…), I think these results will be more relevant and also reveal posts from blogs you don’t ususally see.
As usual John does a fab job of covering the options, choices and competitors. Library Clips rocks! More details about the new bloglines blog search can be found at TechCrunch (Finally! Bloglines Blog Search)