adds a unique CSS class to a single, specific nav menu item.

function my_special_nav_class( $classes, $item ) {

    if ( is_single() && $item->title == 'Blog' ) {
        $classes[] = 'special-class';
    }

    return $classes;

}

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