Google has (finally) added a blog search service. Google Blog Search aims to include "every blog that publishes a site feed (either RSS or Atom). It is not restricted to Blogger blogs, or blogs from any other service."
Google Blog Search comes with the regular and advanced search features offered by Google web search. It supports standard search operators and a few blog specific ones. Additionally at the bottom of each page of search results you will find links, offering the top 10 or 100 results as either Atom or RSS feeds. You can grab these links and subscribe to them in the news aggregator for updates whenever new posts are made that match your query.