Hi Steff,
Its because of the url re-writing and page security. A pageid is associated with the blog and a user must have permission to view that page.
The blog detail page has a real url like this /BlogView.aspx?pageid=x&mid=y&ItemID=z
The blog post is identified by the ItemID
to have a friendly url we create a mapping in the mp_FriendlyUrls table so you can have from a friendly url like my-post-about-something.aspx which maps to the real url as show above. If you move the blog to a new page there is nothing to update this mapping so the request for the friendly url still maps to the page id it was on before and since it is no longer on that page, it can't validate the security to view it any more. New posts will work because they now get the new correct mapping but old posts have borken urls. They can be corrected manually by finding them in the url manager and fixing the real url to have the correct new pageid.
If the blog was on the home page and now you want to use a Feed Manager on the home page instead, then create a new page to be the home page and move it to the root position in the site hierarchy from Administration Menu > Add/Edit Pages
Now you can consume the feed from your blog page which is no longer the home page.
Hope it makes sense.
Joe