It sounds like you've had a bad experience with a former employee, but this is not blackmail. That's just standing up for himself. The OP was working there contingent on his boss's promise of a raise, and both parties knew this, including the boss. In fact, the OP was remarkably generous towards his boss by doing something very foolish: knowingly taking an unlivable wage out of trust that the boss would live up to his word.
Regarding your Wikipedia quote, leaving a job in which you are abused and underpaid is not at all unjustified.
Regarding your Wikipedia quote, leaving a job in which you are abused and underpaid is not at all unjustified.