Actually I beleive the Black Keyboard is to notify the user that you are typing into a secure apple form, while in a 3rd party app. Apple uses the black keyboard when you type your Apple ID password and for other sensivitve fields.
This is useful because a app can have in app purchases. If the app maker creates his own Apple ID and password form, to steal your information, the keyboard would be white. But when Apple asks for your Apple ID, you can be sure it's them asking for it because the keyboard is black.
Edit: Actually as of iOS 7, everyone can acces the black keyboard, by using the UIKeyboardAppearanceDark property. So I guess it doesn't mean it is a secure apple form. Sorry.