Google Operating System informs that Google Blog search ranks the search results by relevancy. The sort by date is also available. However, it is interesting to note the way Google ranks blog posts. 

Positive signals

• links from blogrolls (especially from high-quality blogrolls or blogrolls of "trusted bloggers")
• links from other sources (mail, chats)
• using tags to categorize a post
• PageRank
• the number of feed subscriptions (from feed readers)
• clicks in search results

Negative signals (spam signals)

• posts added at a predictable time
• different content between the site and the feed
• the amount of duplicate content
• using words/n-grams that appear frequently in spam blogs
• posts that have identical size
• linking to a single web page
• a large number of ads
• the location of ads ("the presence of ads in the recent posts part of a blog")

For a better insight read the full post.