You are trying to use the blog in unintended ways. A blog post cannot have the same url as a CMS page. These friendly urls are mapped to real urls by our url re-writer. For any given friendly url it can only map to one real url therfore no duplicates are allowed. So CMS pages, ie pages in the menu like home.aspx map to a real url /Default.aspx?pageid=x and friendly urls for blog posts map to a real url /Blog/BlogView.aspx?pageid=x&mid=y&ItemID=z
If you want to create a blog post and then link it into the main menu you can create a CMS Page Which will create the menu item, and put the fully qualified url to the blog post, not using ~/ but using the fully qualified url like http://yoursite/yourblogpost.aspx
See, because these urls are created for blog posts it is not a good idea for the page corresponding to the url to disappear like with some end date. I have never seen a blog with an end date where the post goes away, that is a strange use case and bad for SEO. Cool urls don't change.