If I were to buy a Blackphone, I would do it because I wanted security, and because I trusted the manufacturer to provide it. The problem with security is that just because my phone appears to be "working perfectly" doesn't mean that somebody isn't eavesdropping on everything I say.
A counterfeit Blackphone, in other words, is completely defective and untrustworthy, no matter how well it appears to work, because my trust in the manufacturer is broken.