Swift 3 (iOS 10)

Transparent UIToolbar

self.toolbar.setBackgroundImage(UIImage(),                                forToolbarPosition: .any,                                barMetrics: .default)self.toolbar.setShadowImage(UIImage(), forToolbarPosition: .any)

Transparent UINavigationBar

self.navigationBar.setBackgroundImage(UIImage(), for: .default)self.navigationBar.shadowImage = UIImage()self.navigationBar.isTranslucent = true

Swift < 3

Transparent UIToolbar

self.toolbar.setBackgroundImage(UIImage(),                                forToolbarPosition: UIBarPosition.Any,                                barMetrics: UIBarMetrics.Default)self.toolbar.setShadowImage(UIImage(),                            forToolbarPosition: UIBarPosition.Any)

Transparent UINavigationBar

self.navigationBar.setBackgroundImage(UIImage(), forBarMetrics: UIBarMetrics.Default)self.navigationBar.shadowImage = UIImage()self.navigationBar.translucent = true

Objective-C

Transparent UIToolbar

[self.toolbar setBackgroundImage:[UIImage new]              forToolbarPosition:UIBarPositionAny                      barMetrics:UIBarMetricsDefault];[self.toolbar setShadowImage:[UIImage new]          forToolbarPosition:UIBarPositionAny];

Transparent UINavigationBar

[self.navigationBar setBackgroundImage:[UIImage new]                         forBarMetrics:UIBarMetricsDefault];self.navigationBar.shadowImage = [UIImage new];self.navigationBar.translucent = YES;

Discussion

Setting translucent to YES on the navigation bar does the trick, due to a behavior discussed in the UINavigationBar documentation. I'll report here the relevant fragment:

If you set this property to YES on a navigation bar with an opaque custom background image, the navigation bar will apply a system opacity less than 1.0 to the image.

Final result

If you want to do it through the entire app you should use the UIAppearance proxy (iOS5+):

UINavigationBar *navigationBarAppearance = [UINavigationBar appearance];navigationBarAppearance.backgroundColor = [UIColor clearColor];[navigationBarAppearance setBackgroundImage:[[UIImage alloc] init] forBarMetrics:UIBarMetricsDefault];navigationBarAppearance.shadowImage = [[UIImage alloc] init];

Docs: https://developer.apple.com/library/ios/documentation/UIKit/Reference/UIAppearance_Protocol/Reference/Reference.html

Article: http://nshipster.com/uiappearance/

Try:

[navBar setBackgroundImage:[UIImage alloc] forBarMetrics:UIBarMetricsDefault];