Make function that retrieves groups associated with a blog use get_col since we just care about the group names.  Expose this function via a filter.  Add xmlrpc function that takes a user and a list of blogs (objects containing a blog_id or actual blog ids) and returns a narrowed down list of those blogs based on whether the user would be able to access them - i.e., they're a member, blog is public, or groups intersect.