Rewrite /blog postname default WP via https://stackoverflow.com/questions/7285532/url-prefix-for-posts-wordpress

rewrite_blog_postname_default_WP.php

<?php
// functions.php
/**
* Rewrite /blog postname
*/
function add_rewrite_rules( $wp_rewrite ){
    $new_rules = array(
        'blog/(.+?)/?$' => 'index.php?post_type=post&name='. $wp_rewrite->preg_index(1),
    );
    $wp_rewrite->rules = $new_rules + $wp_rewrite->rules;
}
add_action('generate_rewrite_rules', 'add_rewrite_rules'); 

function change_blog_links($post_link, $id=0){
    $post = get_post($id);
    if( is_object($post) && $post->post_type == 'post'){
        return home_url('/blog/'. $post->post_name.'/');
    }
    return $post_link;
}
add_filter('post_link', 'change_blog_links', 1, 3);
?>