Xcode Storyboard Shows Blue Frames Around UI Elements. How do I turn this off?

In Xcode 9, turn off Editor->Canvas->Show Bounds Rectangles. The rectangles still don't disappear. Turn it back on, and then turn it off again, and the problem should be resolved.

They are the bounds rectangles for your UI elements - they help you see the size of the various elements where it may not be immediately obvious due to background colours etc.

You can turn them off by going to the Editor menu->Canvas->Show Bounds Rectangles