I wonder why would a person Blackmail another??
Could it be that this person has done something them and thats a way of getting even.
In a job situation probadly "the green eye monster"Ex: your position at work,your salary,your clothes or just being popular.
In a realtionship I have always follow this rule: Always Always no matter what the circumstance part as friends you don't anyone a reason to:blackmail, hate,bad mouth you to others.
Especially those cheating m/f without their partner knowledge when you part from that other person m/f,Do it nicely and with respect leave the book open you ever know when you will need that person for one reason or another.Also remember this is the person you cheated with obvious that they know plenty of info to 'Blackmail" you if they wanted or just to get even if the part of the ways was a harsh one,There is nothing worse than to have a woman scorns.
Those who swinger together with there partner "s" just maybe the person wish to have your open honest lifestyle but because they can't they want to make your life in misery same as there own try and be more friendly to them it might make them forget.