Google's Blogger API is a great solution. It's REST, and is 100% free.
In addition, there is a PHP Version of the Blogger API, so you don't need to worry about writing a wrapper around the REST API.
Don't use AuthSub, it redirects your users to a Google Login page. Instead, use Google ClientLogin. Read the PHP Version of the Blogger API - ClientLogin for details. This uses your login and password to access Blogger to read and write data. It's server-side, but you can still use AJAX to pull data unobtrusively from your server, while your server handles communications to Google.
One possible issue you may encounter is with comments. User Comments will be recorded as if they are from you. Google doesn't yet support creating comments on behalf of the user.
However, you could create the comment, and use your own custom markup to store the name of the commenter in the comment itself. Your application could then parse the name out when displaying the comments. A former colleague of mine actually did something similar in his Picasa API, Comments: Enabled. You can use a similar technique with Blogger.
Sure, there is some work involved, but you're most likely not going to find an out-of-the-box-yet-custom-solution. In my experience, either you adapt to an API, or you develop your own complete, 100% customized solution that conforms exactly to you.
Finally, the code is licensed under the permissive open source BSD license.