Change slug / permalink of WordPress 'posts' post type

functions.php

<?php

/**
 * Rewrite blog posts so they start with /blog
 */

function change_posts_post_type_args( $args, $post_type ) {
  if ( $post_type == "post" ) {
    $args['rewrite'] = array(
      'slug' => 'blog',
 			'with_front' => false
    );
  }

  return $args;
}
add_filter( 'register_post_type_args', 'change_posts_post_type_args', 20, 2 );

function update_posts_post_link( $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', 'update_posts_post_link', 1, 3 );