URL Rewriting if it is a single post / redirecting post – post type

Question

First of all set your permalink structure to /%postname%.

After that add a rewrite rule to rewrite all request like example.com/blog/postname to the appropriate query:

add_action('init', 'blog_front');

function blog_front() {
  add_rewrite_rule('^blog/([^/]+)/?','index.php?name=$matches[1]','top');
}

Now you have to prevent the duplicate url, sending a 301 redirect when example.com/postname is requested:

add_action('template_redirect', 'say_hello_to_google');

function say_hello_to_google() {
  if ( is_main_query() && is_single() && ( empty( get_post_type() ) || (get_post_type() === 'post') ) ) {
    if ( strpos( trim( add_query_arg( array() ), "https://wordpress.stackexchange.com/" ), 'blog' ) !== 0 ) {
      global $post;
      $url = str_replace( $post->post_name, 'blog/' . $post->post_name, get_permalink( $post ) );
      wp_safe_redirect( $url, 301 );
      exit(); 
    }
  }
}

Finally you have to create permalink for your post with ‘blog/’ in front:

add_filter('the_permalink', 'post_permalink_w_blog');

function post_permalink_w_blog( $link ) {
  global $post;
  if ( $post->post_type === 'post' ) {
    $link = str_replace( $post->post_name, 'blog/' . $post->post_name, get_permalink( $post ) );
  }
  return $link; 
}

Last thing to is flush rewrite rules and… test.

Edit

As @Milo pointed out in comments, all of this can be avoided by setting permalink structure to /blog/%postname% and then use the param 'rewrite' having 'with_front' to false for all CPTs and all taxonomies.

'rewrite' => array( 'with_front' => false )

The problem is that most times, in a average WP install with a lot of CPT and taxonomies, there are chances that some of them are registred via 3rd party plugins…
in that case if that plugins register the CPTs/taxonomies not using 'with_front' => false it’s a problem and the solution posted above is the last chance.

In addiction I have to say that standard categories and tags are registered with ‘with_front’ = true, so using this alternative method also standard category and tag must be re-registered with with_front param setted to false.