The fifth amendment defense was overturned in the Boucher case because: "A District Court judge agreed with the government, holding that, given Boucher's initial cooperation in showing some of the content of his computer to border agents, producing the complete contents would not constitute self-incrimination." [1]. Without Boucher having given up the names and titles of his files, it is entirely reasonable to expect that the fifth amendment would hold when testimony is asked without knowing what is inside the encrypted space.

[1] https://secure.wikimedia.org/wikipedia/en/wiki/United_States...