There are over 35 million and there is no list one can use to locate them. The WordPress.com search engine indexes post, pages, and comments content (body text). Unless entered as text in those contents, blog name, post titles, and post/comment author names are not indexed. http://en.support.wordpress.com/wordpresscom-search/
For Google searches use this format and replace “keyword” with whatever word you wish:
FYI – I discovered something…. If I go into the Blogs that I follow and choose edit, I get a search box inwhich I can enter a wordpress URL. Voila !! I am now able to follow a specific blog as long as I know the URL.