Fix nav menu issues with custom post types. If, when you link to a custom post type in your nav menu, the blog page also highlights the nadd the following to your functions.php, where "menu-portfolio" is the name of the nav menu item you need to fix.

WordPress - fix nav menu highlighting issue when using custom post type

function fix_portfolio_parent($menu){
  global $post;
  if ( 'portfolio' == get_post_type() )
  {
    $menu = str_replace( 'current_page_parent', '', $menu ); // remove all current_page_parent classes
    $menu = str_replace( 'menu-portfolio', 'current_page_parent menu-portfolio', $menu ); // add the current_page_parent class to the page you want
  }
  return $menu;
}
add_filter( 'nav_menu_css_class', 'fix_portfolio_parent', 0 );