The blog is a something that one person or group of persons (authors) exposing to the public. So in some cases this material may need commentary from other people than authors. This is a option to the blog settings, when it's enabled (default) the home page of the blog (via permaLink) will offer a post form to make a comment. When making a comment the author's details can be recorded and re-used based on cookie; comment poster may control that option. Once comment is posted it can be read in same place (permaLink) as in discussion group.