Menu — How to add “current-menu-grand-ancestor” css class

Something like this will help you filter a menu item and add a custom class conditionally. ( Untested )

function add_nav_class( $classes ) {

    if ( class_exists( 'current-menu-ancestor' ) || $item->title == 'blog' )  {
        $classes[] = 'current-menu-grand-ancestor';
    }

    return $classes;

}

add_filter( 'nav_menu_css_class', 'add_nav_class', 10, 2 );