Snip um die Navigationsleiste komplett transparent zu machen:

let visualEffectView   = UIVisualEffectView(effect: UIBlurEffect(style: .light))
UINavigationBar.appearance().shadowImage = UIImage()
UINavigationBar.appearance().backgroundColor = UIColor(red: 0.0, green: 0.0, blue: 0.0, alpha: 0.0)
UINavigationBar.appearance().isTranslucent = true
UINavigationBar.appearance().addSubview(visualEffectView)
UINavigationBar.appearance().setBackgroundImage(UIImage(), for: .default)