I've got a UIPickerView as the inputView of a UITextField and can successfully set the background on iOS 8:
statePicker.backgroundColor = [[UIColor blackColor] colorWithAlphaComponent:0.25f];
But on iOS 7, I'll be damned if every single thing I've tried from other answers just results in this:

Thoughts?
Verify your UITextField does not have his Appearance value set to Dark in the attributes inspector, just below the Keyboard Type setting. If so set it back to Default.
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