Yeah I found the method! (and it was so fun!) (and got me killed one time)
I think that is indeed The Gisette Blackmail, considering how the Blain blackmail also pops up in a specific conversation and not as an option during negotiations. I'm now wondering if the (very obvious and non-critical) Lyon secret from W1 can pop up somewhere... That's the only one I can think of that I didn't see have a follow-up somewhere.