Rewrite - add /blog/post-name to posts

new-rewrite.php

/**
 * Add new rewrite rule
 */
function create_new_url_querystring() {
    add_rewrite_rule(
        'blog/([^/]*)$',
        'index.php?name=$matches[1]',
        'top'
    );
    add_rewrite_tag('%blog%','([^/]*)');
}
add_action('init', 'create_new_url_querystring', 999 );
/**
 * Modify post link
 * This will print /blog/post-name instead of /post-name
 */
function append_query_string( $url, $post, $leavename ) {
    if ( $post->post_type == 'post' ) {
        $url = home_url( user_trailingslashit( "blog/$post->post_name" ) );
    }
    return $url;
}
add_filter( 'post_link', 'append_query_string', 10, 3 );