Custom navigation bar image + translucency effect not working for iOS 7

Question

My iOS 7 app uses a custom background image for the navigation + status bar. It works fine but when the content of a view is scrolled up behind the navigation/status-bar it will not get blurred the way like when not using a custom image. Instead the content appears still quite sharp. Is there a way to add the blur effect?

I simply us in my delegate:

 [[UINavigationBar appearance] setBackgroundImage:[UIImage imageNamed:@"[email protected]"] forBarMetrics:UIBarMetricsDefault];
 [[UINavigationBar appearance] setBackgroundImage:[UIImage imageNamed:@"[email protected]"] forBarMetrics:UIBarMetricsLandscapePhone];