Agreed refactoring method signatures will cause tests to break, but they should be relatively trivial to fix.

Black box tests are not limited to behavioral or system integration testing. you can black box at the unit level. In fact if you are doing proper TDD you will write the tests first, which help to define the interface of the class under test, make sure the test fails to ensure you are testing for something, and then finally write the implementation to make the test pass. Such an approach implies it is black box as you've defined your interface before your implementation.

Remember you are testing what it does, not how it does it.