Swift version, works perfectly globally:
func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
UIBarButtonItem.appearance().setTitleTextAttributes([NSForegroundColorAttributeName:UIColor.clearColor()], forState: UIControlState.Normal)
UIBarButtonItem.appearance().setTitleTextAttributes([NSForegroundColorAttributeName:UIColor.clearColor()], forState: UIControlState.Highlighted)
return true
}