I do not believe you have to know what happens inside. And concept of blackbox in engineering actually comes from this insight. But the approach to solve a problem is more important. That is why debugging a code is blamed in testing.
When I say, candidate should know what happens inside, I do not mean he should be knowing all minute detail of it, but a generic high level understanding of tools/framework being used is expected so if required one would not hesitate to go under the hood and try to find problem.
Black boxes are bad. Yes, you can build things. But you don't know how it why they work, and when they stop working you don't know why because you never knew why it worked in the first place.