If you wanted to spice up your modally presented view controllers for iOS 8 users by adding a UIVisualEffectView you will likely end up with the this.

Awful animation

let modalViewController = ModalViewController()
self.presentViewController(modalViewController, animated: true, completion: nil)

The reason for this is that the view of the presenting view controller is removed from the view stack.
Luckily Apple has us covered with a new UIModalPresentationStyle called UIModalPresentationOverFullScreen.

A view presentation style in which the presented view covers the screen. The views beneath the presented content are not removed from the view > hierarchy when the presentation finishes. So if the presented view controller does not fill the screen with opaque content, the underlying content shows through.

Available in iOS 8.0 and later.
Apple Docs

As wanted the views beneath the presented view controller remains on the view stack.
So with almost no effort at all we can get the UIVisualEffectView to work as intended on the modal view controller.

let modalViewController = ModalViewController()
modalViewController.modalPresentationStyle = .OverFullScreen
self.presentViewController(modalViewController, animated: true, completion: nil)

All fine here

Source code for ModalViewController is as follows:

import UIKit

class ModalViewController: UIViewController {
    override func viewDidLoad() {
        self.view.backgroundColor = .clearColor()

        let visuaEffectView = UIVisualEffectView(effect: UIBlurEffect(style: .Light))
        visuaEffectView.frame = self.view.bounds
        visuaEffectView.autoresizingMask = .FlexibleWidth | .FlexibleHeight

Have a lovely day :)