active menu in post page

Question

The below filter adds your class based on title, you could also do this by post ID if you’d like: $item->object_id == 123

function wp_nav_parent_class( $classes, $item ) {

    if( is_singular('post') && $item->title == "Your Menu Item Title Here" )
        array_push($classes, 'current_page_parent');

    return $classes;
}
add_filter('nav_menu_css_class', 'wp_nav_parent_class', 10, 2);

If is_singular('post') does not work try is_single(), but it should work. Just replace the conditional string with the menu item title of your choice.