we don't have URL for each blog post (saved in the database). However you can create URL for each blog post dynamically.
In the BaseBlogPart class, you can find FormatBlogPostUrl method, which will return you URL of the specified blog post. So basically while you're populating RssChannel with a RssChannelItem objects, you can call something like this:
RssChannelItem rssItem = new RssChannelItem();
rssItem.Link = UrlFormatter.ResolveServerUrl(this.FormatBlogPostUrl(blogPost));