If you are interested only in new posts, you can look in blogs' RSS feeds. They are nearly always in default locations.

Or you could parse the URL - I had a similar task some time ago, and I went with URLs - Blogger and Typepad are consistent; WordPress depends on the blog, of course, but you could figure out several most popular patterns (e. g. /yyyy/mm/dd/posttitle, /id-posttitle) and get like 90% of all blogs right.

Or maybe, just maybe, you could use some third parties that have already figured it out via RSS - maybe Technorati?