This will print /blog/post-name instead of /post-name for posts

snippet.php

<?php
// Download this snippet as a plugin and more at: https://wordbits.io/snippet/add-url-prefix-for-blog-posts/
// Created by: Mark Vi
?>

<?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
 */
add_action( 'init', 'redefine_post', 1 );
function redefine_post() {
    register_post_type( 'post', array(
        'labels' => array(
            'name_admin_bar' => _x( 'Post', 'add new on admin bar' ),
        ),
        'public'  => true,
        '_builtin' => false,
        '_edit_link' => 'post.php?post=%d',
        'capability_type' => 'post',
        'map_meta_cap' => true,
        'hierarchical' => false,
        'rewrite' => array( 'slug' => 'blog' ),
        'query_var' => false,
        'has_archive' => true,
        'supports' => array( 'title', 'editor', 'author', 'thumbnail', 'excerpt', 'trackbacks', 'custom-fields', 'comments', 'revisions', 'post-formats' ),
    ) );
}