I think one of the best ways to become a competent black-hat hacker/pen-tester would be to know the white-hat techniques inside out. To become knowledgeable about a system's security (including vulnerabilities) would allow you to identify and exploit vectors a lot more efficiently. In fact, it's probably pretty critical.
If you were to look at MOST of the questions on security.SE you could interpret them as belonging to some black-hat origin. Take mine for example: Testing a black-box for sound authentication session config I've asked how I can interrogate a session which could then lead me to exploiting a poorly designed authentication system.
For the record, this is purely for ethical pen-testing reasons, but what if it isn't? Is it up to the user of security.SE to judge this if there were a more blatant attempt at learning for the wrong reasons? Or is that not our responsibility here?