I think this is a bug in the documentation. You should file it at bugreport.apple.com.
Everything I can see after a bit of quick research suggests what you are seeing is how it always has behaved, and my own testing shows it too.
The alpha of a view is applied to all subviews.
Perhaps all you need is [[UIColor blackColor] colorWithAlphaComponent:0.5] but if not you will need to make the view a sibling instead of a child.