When you rename a page or change its url a 301 redirect is created from the old url to the new url. This is done to help SEO in case users have bookmarked the old url or search engines have indexed the old url.
My guess is that there is a redirect for blog.aspx or it is in use already. I would look for it under Administration > Advanced Tools > 301 Redirects and also under Url Manager, and delete the redirect or existing url if it is not used or the redirect is not intended.
Hope it helps,
Joe