Adamah:
I used the term blackmail loosely because ordinary (non legalistic) person's understand that it is obtaining something from another person without using physical force. And ordinary people understand that it is bad. Poetic licence if you will.
Actually in a legal sense it is neither extortion or blackmail. The slave knew the rules when he took a wife from his master's household. So, the Master wasn't doing anything wrong legally. Now if the Master gave a woman in his household to the slave and certified it in writing and then decided to change his mind he would have a problem with the men in the gates. The Watchtower also makes the claim that when you got baptized in recognition of the "spirit led organization" that you knew what you were getting into. That's how they can deny any wrongdoing by disfellowshipping someone who leaves.
Even if the Master is nice he isn't noble and endearing. And to be sure this loop hole was exploited. It was a legal way to retain slaves past the date of release and calls into question the whole system of slavery.
And what would you expect of a Master (Jehovah) who is so unforgiving that he had his only begotten son die from torture?