UIVisualEffectView creates a grey box instead of blurring

I'm using a UIVisualEffectView to blur a section of the screen in SpriteKit like so:

let blur:UIBlurEffect = UIBlurEffect(style: UIBlurEffectStyle.Dark)
let UIEffectView:UIVisualEffectView = UIVisualEffectView(effect: blur)
UIEffectView.frame = CGRectMake(0, 0, frame.width, frame.height * 0.15)
self.view?.addSubview(UIEffectView)

But it appears as a grey shaded area instead of a blur. Strangely, it blurs whilst the GameCenter notification shows at the start, saying "Welcome back, (player)" - but once the notification disappears, it goes back to being a grey box.

Same applies to when you go into the multi-task view in iOS9, the preview has the section blurred, but when you go into the app, it reverts to being a grey shaded area again.

Any ideas why this is happening? It does the same in the simulators as well as physical hardware.