Blackmail has something in common with transactions that become repugnant when money is added

Blackmail isn't what I usually mean when I speak of a repugnant transaction, because it isn't a voluntary transaction that third parties wish to prevent, it's a crime that someone assaults someone else with. But, just as some praiseworthy transactions (like donating a kidney to someone who needs a transplant) can become repugnant when money is added (both demanding compensation and compensating the donor of a kidney for transplant is a crime almost everywhere), the crime of blackmail involves combining actions that are otherwise legal but that together are criminal.

For example, if someone knows something about you that you would like to conceal, it is quite often legal for them to reveal it to interested parties, or even in a biography of you they might write (e.g. your past arrests, affairs, political affiliations and contributions, etc.). It would also be quite legal for you to approach someone and commission them to write something about you, including something over which you might have editorial control. But if someone proposes to combine these things, by threatening to write bad things about you unless you pay him money, that is in many cases the criminal act of blackmail.

More on blackmail and it's subtleties (e.g. I can't demand that you pay me if you don't want me to reveal that you are a thief, but I could threaten to report you the police if you don't pay me for something you stole from me...) in this interesting column at the Washington Post's Volokh Conspiracy:
Blackmail is surprisingly hard to define