How to add a blur mask with a custom shape over the dynamic camera view in Swift?

I use Swift to develop an iOS application with a camera function, and this requires a blur layer above the camera image with a hole in the middle, as shown in the image below.

I tried several methods, but none of them added a blur effect without covering the hole. I did not find working solutions, spending time on Google.

Can someone give some clues on how I can only erase the non-transperant portions of a PNG image that is attached to the image view?

Methods I tried:

• Use iOS 8's native blur effect

  let blurEffect = UIBlurEffect(style: UIBlurEffectStyle.Dark)
  let blurEffectView = UIVisualEffectView(effect: blurEffect)
  maskImage.addSubview(blurEffectView)
  

• Use the filter "CIGaussianBlur"

  var imageToBlur = CIImage(image: image)
  var blurfilter = CIFilter(name: "CIGaussianBlur")
  blurfilter.setValue(imageToBlur, forKey: "inputImage")
  var resultImage = blurfilter.valueForKey("outputImage") as! CIImage
  var blurredImage = UIImage(CIImage: resultImage)
  self.maskImage.image = blurredImage
  

The visual effect I want to have is: