Yahoo recently integrated blog search with Yahoo! News Search. When you search on Yahoo! News, you see the top 4 blog results in a sidebar on the right. If you are looking for complet set of results, you must click on "More Blog results" to see additional blog posts, Flickr photos and links from My Web.If that sounds a little complex, you can use the form here to access yahoo blog search directly skipping the Yahoo news search page. Just enter the search query and click the search button.
Here's the HTML source of the Yahoo Blog Search box:
<form action="http://blog.news.search.yahoo.com/blog/search" method="get">
<input type="text" name="p"/>
<input type="submit" value="Search Blogs"/>
</form>