set up 404 redirects from /%postname%/ to /blog/%category%/%postname%/

structure-redirects.php

<?php
/** set up redirects from old structure */
function aly_post_redirects() {

	if ( ! is_user_logged_in() )
		return;

	if ( is_404() ) :

		$slug	= $_SERVER['REQUEST_URI'];
		$slug	= str_replace( '/', '', $slug );

		$item	= get_page_by_path( $slug, OBJECT, 'post' );

		if ( ! $item )
			return;

		$post_link	= get_permalink( $item->ID );

		if ( ! $post_link )
			return;

		wp_redirect( esc_url_raw( $post_link ), 301 );
		exit;

	endif;

}

add_action ( 'template_redirect', 'aly_post_redirects', 1 );