One so-called paradox of blackmail concerns the fact that “two legal whites together make a black.” That is, it is licit to threaten to reveal a person’s secret, and it is separately lawful to ask him for money; but when both are undertaken at once, together, this act is called blackmail and is prohibited. A second so-called paradox is that if the blackmailer initiates the act, this is seen by jurists as blackmail and illicit, while if the blackmailee (the person blackmailed) originates the contract, this is commonly interpreted as bribery and is not illicit.
But these are paradoxes only for legal theorists innocent of libertarian theory. The authors use that perspective to reject the claim that blackmail should be unlawful. If this act were legalized, then both paradoxes would disappear, precisely their contention.