First I would say I don't recommend doing that unless you clone the blog into your own private feature with different namespace, tables and procs etc. The blog is subject to change in future versions and your code may break as a result. At some point we will expose a web service using the MetaWebBlog api and then it will have an interface you can rely on not changing and therefore could code against it with confidence. Some of the work for this is done already thanks to Tom Opgenorth but I need to review it and update it as even the work he did is out of date with the current version of the blog.
Now to answer your question, I suspect you are not creating the friendly url mapping. If you study the code in /Blog/EditPost.aspx.cs you should be able to figure it out. The real blog page is /Blog/Viewpost.aspx?pageid=x&mid=y&ItemID=z but we use friendly urls that map to this url.
Hope it helps,