
If I'm keeping information to myself that is pertinent to the task, that may mean I don't have enough confidence yet that the specific task can be successfully accomplished by the other person. That necessary inside info is what should go into the black-box.
Maybe the person is a new contact I've never interacted with before or maybe I'm not sure that they truly have enough time. If that's the case then it's a signal to me that I need to make the black-box smaller. If the task is finished successfully then my confidence builds and I'm able to hand-off a bigger black-box task the next time around.
But if I don't black-box I can never build my trust relationship up enough to truly hand-off tasks completely.
When you really don't need your reader to do anything, leave him off the hook in your email like this:
Dear Joe,
(No Action Requested From You)
I got your reports and they are exactly what we needed.
See you tomorrow at the customer's site for the presentation.
Cordially,
Jonathan.