The “proper way” to do this would be to respond with a 301 Permanently Moved at the old url. Assuming you’re using apache, a mod_rewrite. Something like this in an .htaccess file in the docroot for http://website/blog/ would work:
RewriteEngine on
RewriteRule ^feed/$ http://blog.website/feed/ [R=permanent,L]
This tells apache to respond with something similar to the following:
HTTP/1.1 301 Moved Permanently
Location: http://blog.website/feed/
Any decent feed reader will recognize the 301, redirect to the new url, and update itself to use the new url in the future.
Check out the mod_rewrite docs and the status code section of the HTTP spec for more info.
