He wouldn't have had a reason to blackmail her period IF she cut it off after the blackmail started. The only reason that he would blackmail her is if she cut off her relationship with him and he wasn't happy about that. In The Rival quest he mentions how the MC showed up and started stealing all of the woman, so I am pretty sure that Victoria stopping her affair with Eckhardt had something to do with the MC and him being younger than him and a bigger cock.
My take on how it went down...
E: "I have the evidence of your unfaithfulness, slut. Those panties you promised to only wear for your husband. Not that they ever did anything for him. Start paying me or I'll tell him about us."
V: "He'd never believe you!"
E: "I'll say you came on to me. Which, to refresh your memory, you did. And that I was weak to your temptations, but I can't live with the shame of sleeping with another man's wife any more."
V: "You wouldn't!"
E: "I would. And after he divorces you I'm sure you'd make a fine tavern wench again, even now. I'd even come see you there. Regularly."
V: "I'll never fuck you again!"
E: "I guess you're not as much of a slut as I thought. But you'll still pay me every week to keep your secret, and your lifestyle. Minus the sex, of course...you were so desperate for a good, hard pounding when you came to me. And don't bother trying the same thing with any of my men, either. They know what'll happen if they get caught with any local girls other than Gwynneth, let alone you."
V: (storms out)