I’ve gotten blackmail options three times so far - one for Jaslen’s secret, one for Avalie’s, and one for Zarad’s (for Jaslen, I only needed to see one scene - for Zarad, I needed his confession to get the option.) For both options, it happened at the end of the day’s negotiations, where I was asked if I wanted to blackmail the character whose secret I knew. For Avalie, I think I only got the option when she wasn’t my ally.