While Posterous also has a convenient import tool to go from Wordpress/Blogger/Tumblr/etc., there's no tool that lets you go from Posterous to Blogger and it required a little bit of a hack to do this due to a "Sorry, the import failed due to a server error. The error code is bX-q5k3d4"
1. Get a Wordpress account and use the Import tool with Posterous. You'll need to enter your Posterous credentials.
2. Export your Wordpress blog that contains all of your Posterous posts. Doing this will create an xml file that isn't compatible with the Blogger import tool and gives you that bX-q5k3d4 error code.
3. Use the wordpress2blogger (http://wordpress2blogger.appspot.com/) app to convert the Wordpress. Note that this app restricts you to files smaller than 1MB. What you can do is import small portions of your blog with the Wordpress exporter in step 2.
4. Import the fixed xml file via the Settings -> Blog Tools -> Import Blogs tool in Blogger