only update titles of single posts

If you want to wrap post title of posts only, then you should use get_post_type() to check post type of post. You can take reference from below code, it will applied on “post” post type only. and want to apply on post single page then you have to add is_single() with get_post_type() condition.

function apply_titles($title, $id)  {
     if('post' == get_post_type($id) && is_single())
     {
        return "* ".$title." *";
     }
     return $title;
}
add_filter('the_title', 'apply_titles', 10, 2);