Technorati has added a (beta) blog finder service. Previous searches for terms or tags returned matching posts. This one helps you find blogs that frequently post on a topic, as determined by tags associated with the blog. It looks like blogs were initially assigned tags from those on its past posts. You can manually associate up to 20 tags with your own blog if you have “claimed” it. Query results are presented in order of blog authority as measured, I believe, by the number of in-links. This definitely looks worthwhile.
The query capability is quite limited, however. It looks like you can only issue simple queries (e.g., blogs tagged with AI) or disjunctive queries (e.g., blogs tagged with AI OR Artificial Intelligence. Operators for conjunction and negation are not provided.