What does it mean if something is black box?
The short version is that it does something, but don’t ask how. Imagine I have you a black box one day. Now, this magic black box could take any date you entered and give you the lotto numbers for that date. Maybe you could type a username in and their password would be presented to you. You’ll never get to see how or why, but the magic black box will do.
All your testing on this magic black box is done with no understanding of and no need to understand what’s inside. If there’s a tiny green goblin creature living in it that returns your results, it doesn’t matter. If you put X in, you’ll get Y out. The great part about this is the simplicity and service level agreement that comes with it. If I tell you that 99.99% of the time the service will be available, that’s all you need to know.