Can you post a link to your site?
Are you sure that your blog page is not protected by roles and allows all users to view it?
Once you create a blog and make a blog post you should not move the blog instance to a different page. If you do that it will cause this error because the friendly url for the post in Url Manager will have the wrong pageid in the real url that it maps to. So if you did that you may need to edit the urls for existing posts (that were created when the blog was on the original page) to correct the pageid.
Hope it helps,