I agree, but I don't think the real point is to be able to read behind the keyboard. A text input box would be available just like on the current keyboard.
I think everyone is thinking about the full screen finger keyboard, not the pop-up stylus keyboard, so this doesn't really apply - the keyboard already provides it's own text box and cursor navigation features, separate from the application.

However you are correct - if you shade it darker, and it's on a dark application, it might be harder to see the keys. It's rare that a screen will be so dark that this would be an issue (near black?) but still something that must be considered.