Correct interface design and mocking will allow you to black-box all your unit-tests.
That's a big statement, but I'm willing to be proven wrong. (Yes, that's a challenge.)
I basically agree, with a moderate number of caveats :-)
- Legacy code. I'm going to carry on adding white-box unit tests to test free code that's not been designed with testing in mind.
- Code clarity. Sometimes I think the code remains clearer with a couple of private methods than it does by factoring out a separate support class. If I think that I'll quite happily write white box tests if I need to tweak them.
- Can TDD ever be black box? Since the next test is driven both by the spec and by your knowledge of the implementation you can argue that it's always white box. The kind of unit tests that TDD produce are very different from the sort of unit tests a pure specification based approach might take (see discussion on the TDD list for example).