Whether the comments box shows up on the blog post or not depends on how the reader gets there. Any given blog post can have two or three URLs. If you reach it by clicking an archive link or a category link, the URL will have the archive or category info in it (depending on how you use permalinks of course).
These are not the actual blog-post URL. If you click on the blog title, it will open the blog page and the comments box will be there.
I hope that helps
