Battelle posts a review of Sphere in which he praises it for its relevance ranking. AFAIK, Sphere is the first blog search engine, other than Google, to use something other than simple link counting (+alpha) to provide relevance. Doing relevance ranking per post is a natural way to help spam drop to the bottom of the results list. However, the problems that I'd like to see solved is: filter out spam in the time ranked results, and: search for and rank blogs - not posts.
I'm surprised that no-one's yet come up with a really strong find-a-blog solution. Does this mean there is no user need (other than mine) for this feature? Since its launch, Technorati's blog search feature hasn't had much buzz (10s of posts a day):
Though a search for the URL may not be the fairest measure of popularity here. Alexa reports that 98% of traffic to Technorati is to the front page with no mention of other specific search pages (tags, blogs, etc.)