I think reading and writing test code is harder than the underlying code itself. You must know both the desired behavior of the code-under-test and whether the test correctly stresses that behavior. And then if you're working on anything more complicated than a single unit test you must also make sure that it doesn't blow up anything adjacent to it (preserve state).

I think enforcing black box testing is the best way to get useful tests out of both humans and robots. They must not know the internals, or it will lead them to do bad things.