According to the Google Operating System blog, unlike most blog search engines, Google Blog Search ranks the results by relevancy. According to a new patent Google uses indicators to reflect the quality of a blog or of a blog post.
To rank the search results, Google combines a quality score obtained by mixing those signals with a relevance score (IR score) that depends on the query. “The IR score may be determined based on the number of occurrences of the search terms in the document. The IR score may be determined based on where the search terms occur within the document (e.g., title, content, etc.) or characteristics of the search terms (e.g., font, size, color, etc.). A search term may be weighted differently from another search term when multiple search terms are present. The proximity of the search terms when multiple search terms are present may influence the IR score.” (the quote was slightly altered for clarity)
Google Operating System outlines the positive and negative systems Google blog search uses. However, to summarize, focus on providing your readers with high quality, relevant content and a great experience that, obviously, isn’t done by an automated bot.