Swift - fading the navigationBar to a colour

fade.swift

import UIKit

/// Won't work - will expand the colour from Left hand corner out
self.navigationController?.navigationBar.barTintColor = nil
UIView.animateWithDuration(1.0) {
    self.navigationController?.navigationBar.barTintColor = UIColor.blueColor()
}

///Works
self.navigationController?.navigationBar.barTintColor = UIColor.whiteColor()
UIView.animateWithDuration(1.0) {
    self.navigationController?.navigationBar.barTintColor = UIColor.blueColor()
}