On my iPhone it does. It goes from a light grey (normal keyboard) to a dark grey (system password entry keyboard).
Try it in a few more apps it's the choice of the app dev so basically random.
Not that a user should have to know "only enter your password into the black keyboard", which can also be faked mind.
Yes, I know about UIKeyboardAppearance and how apps can influence that, this isn't what I am talking about. When I say "system password" dialog, I am talking specifically about requests for my iCloud/iTunes/phone password. Unfortunately I can't seem to trigger it now, but I distinctly remember in the past the keyboard looked slightly different than the dark keyboard theme. Maybe I am wrong!