How often should I mention my character’s blackmail material?

How often should I mention my character’s blackmail material?

Character A is going to sabotage character B at the end of the novel, resulting in the permanent breakdown of their relationship. The course of the novel is Character A creating a false sense of security for Character B in order to make the sabotage more dramatic and impactful.

In the first chapter, I show character A obtaining the blackmail material that he uses for his sabotage. It was a short, 150 word section of him literally capturing the material, out of about 10k worth of words.

I’ve never written anything that uses Chekhov-like devices, so I’m not sure how often I should mention or at least draw attention to the material? I’m concerned that if it comes out of nowhere at the end, people will be like “what? I don’t remember that?” but hammering it into every chapter seems a bit forced.

If you have any examples of where something like this has been pulled off well, please do recommend it to me! Otherwise, general advice is much appreciated.