From the FAQ:
Blog Search is Google search technology focused on blogs. Google is a strong believer in the self-publishing phenomenon represented by blogging, and we hope Blog Search will help our users to explore the blogging universe more effectively, and perhaps inspire many to join the revolution themselves. Whether you're looking for Harry Potter reviews, political commentary, summer salad recipes or anything else, Blog Search enables you to find out what people are saying on any subject of your choice.
Your results include all blogs, not just those published through Blogger; our blog index is continually updated, so you'll always get the most accurate and up-to-date results; and you can search not just for blogs written in English, but in French, Italian, German, Spanish, Chinese, Korean, Japanese, Brazilian Portuguese and other languages as well.