How can I modify the slug for the default wp posts without affecting other posttypes?

example: www.example.com/slug-of-post/ should become www.example.com/blog/slug-of-post/

I made a few custom post types where I rewrote the slug by using 'rewrite' => array('slug' => 'portfolio'), in the function.php file.

Result: www.example.com/post-type-slug/slug-of-post

I need the same result but for the default posts.

Thanks.