The setup is simple.
You can add blur effect by UIBlurEffect and UIVisualEffectView:
@IBAction func blur(_ sender: Any) {
let darkBlur = UIBlurEffect(style: UIBlurEffectStyle.dark)
let blurView = UIVisualEffectView(effect: darkBlur)
blurView.frame = bg_imagview.bounds
blurView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
bg_imagview.addSubview(blurView)
}