Apple has introduced new classes UIVisualEffectView and more to add translucency and blur effect on views from iOS 8.0 release.

Here how you can use it to add a blur effect to navigation bar or any other UIView:

Swift 5

func addBlurEffect() {    let bounds = self.navigationController?.navigationBar.bounds    let visualEffectView = UIVisualEffectView(effect: UIBlurEffect(style: .light))    visualEffectView.frame = bounds ?? CGRect.zero    visualEffectView.autoresizingMask = [.flexibleWidth, .flexibleHeight]    self.navigationController?.navigationBar.addSubview(visualEffectView)            // Here you can add visual effects to any UIView control.    // Replace custom view with navigation bar in the above code to add effects to the custom view.}

Objective C Code:

- (void) addBlurEffect {    // Add blur view    CGRect bounds = self.navigationController.navigationBar.bounds;    UIVisualEffectView *visualEffectView = [[UIVisualEffectView alloc] initWithEffect:[UIBlurEffect effectWithStyle:UIBlurEffectStyleLight]];    visualEffectView.frame = bounds;    visualEffectView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;    [self.navigationController.navigationBar addSubview:visualEffectView];    self.navigationController.navigationBar.backgroundColor = [UIColor clearColor];    [self.navigationController.navigationBar sendSubviewToBack:visualEffectView];    // Here you can add visual effects to any UIView control.    // Replace custom view with navigation bar in the above code to add effects to the custom view.}

UPDATE:

If you find that after adding blur effect on navigationBar, navigation buttons are not visible then add below line after adding blurView code.

Swift:

self.navigationController?.navigationBar.sendSubview(toBack: visualEffectView)

Objective C:

[self.navigationController.navigationBar sendSubviewToBack:visualEffectView];

Swift 4

extension UINavigationBar {    func installBlurEffect() {        isTranslucent = true        setBackgroundImage(UIImage(), for: .default)        let statusBarHeight: CGFloat = UIApplication.shared.statusBarFrame.height        var blurFrame = bounds        blurFrame.size.height += statusBarHeight        blurFrame.origin.y -= statusBarHeight        let blurView  = UIVisualEffectView(effect: UIBlurEffect(style: .light))        blurView.isUserInteractionEnabled = false        blurView.frame = blurFrame        blurView.autoresizingMask = [.flexibleWidth, .flexibleHeight]        addSubview(blurView)        blurView.layer.zPosition = -1    }}

Usage

navigationController?.navigationBar.installBlurEffect()

Noted: on iOS 11, function sendSubviewToBack does not work normally. In order to achieve that, we should use zPosition to place the blur effect view under other views.

self.visualEffectView.layer.zPosition = -1;

Objective-C code

[self.navigationController.navigationBar setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];    self.navigationController.navigationBar.shadowImage = [UIImage new];    self.navigationController.navigationBar.barTintColor = [UIColor whiteColor];    self.navigationController.navigationBar.backgroundColor = [UIColor clearColor];    self.navigationController.navigationBar.translucent = YES;    // Add blur view    CGRect bounds = self.navigationController.navigationBar.bounds;    bounds.size.height += 20;    bounds.origin.y -= 20;    _visualEffectView = [[UIVisualEffectView alloc] initWithEffect:[UIBlurEffect effectWithStyle:UIBlurEffectStyleLight]];    self.visualEffectView.frame = bounds;    self.visualEffectView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;    self.visualEffectView.userInteractionEnabled = NO;    self.visualEffectView.layer.zPosition = -1;    [self.navigationController.navigationBar addSubview:self.visualEffectView];    [self.navigationController.navigationBar sendSubviewToBack:self.visualEffectView];

Swift 4 code

  self.navigationController?.navigationBar.isTranslucent = true    self.navigationController?.navigationBar.setBackgroundImage(UIImage(), for: .default)    let visualEffectView   = UIVisualEffectView(effect: UIBlurEffect(style: .light))    var bounds = view.bounds    bounds.size.height += 20    bounds.origin.y -= 20    visualEffectView.isUserInteractionEnabled = false    visualEffectView.frame = bounds    visualEffectView.autoresizingMask = [.flexibleWidth, .flexibleHeight]    self.navigationController?.navigationBar.addSubview(visualEffectView)    visualEffectView.layer.zPosition = -1