100% opacity UILabel over a 50% opacity background (UIView?)

Just set the background color to be semitransparent:

view.backgroundColor = [[UIColor blackColor] colorWithAlphaComponent:0.5f];

Or, in Swift:

view.backgroundColor = UIColor.blackColor().colorWithAlphaComponent(0.5)

Or, Swift 3:

view.backgroundColor = UIColor.black.withAlphaComponent(0.5)

Note that, in this particular case, UIColor(white: 0, alpha: 0.5) is more concise, but colorWithAlphaComponent will work in general.