WordPress.com supports multiple blogs per account. However this is not fully implemented in the xml-rpc end. For example, if you have two blogs, blogA.wordpress.com and blogB.wordpress.com, then
1) The xml-rpc call to list blogs (eg: blogger.getUsersBlogs) correctly returns the two blogs (along with the blog ids).
2) List of posts and blog categories (either blogger, metaweblog or mt api) does not work for multiple blogs - it always returns the data for the first blog, even if you provide the id of the second (or Nth) blog when making the API call.
metaWeblog.getCategories(blogId_A, username, password)
metaWeblog.getCategories(blogId_B, username, password)
returns the same data i.e. categories for blogId_A. The same applies to getRecentPosts.