Voir ici :

http://wpchannel.com/supprimer-blog-permaliens-wordpress-3-0-multisite/

Function.php

add_filter("pre_update_option_category_base","cc_remove_blog_slug");
add_filter("pre_update_option_tag_base","cc_remove_blog_slug");
add_filter("pre_update_option_permalink_structure","cc_remove_blog_slug");

function cc_remove_blog_slug($tag_cat_permalink){

if(!preg_match("/^\/blog\//",$tag_cat_permalink))
return $tag_cat_permalink;

$new_permalink=preg_replace ("/^\/blog\//","/",$tag_cat_permalink );
return $new_permalink;

}