assuming you are on apache and don't have access to the server config file, you probably want something like this in your .htaccess file of the root directory: RewriteRule ^blog/(.*)$ http://example.com/newname/$1 [R=301,L]
however, where you put this directive and whether it is correct as is depends on what else you have going on in your configuration and where.
there may be a WP plugin that would handle these external redirects for you. if you need more Apache mod_rewrite help, you should post your relevant directives there for discussion.