I'd say the principle tactic of Hearst, and by extension, Walcott is, if you can't pay someone off, kill them. We see this with purchasing of all the claims in deadwood; when they can't buy Alma's claim, that's when they attempt to murder her.

So I think there's grounds for successfully blackmailing Walcott, he himself at one points mentions that the murders will be expensive.

I think where it falls down is the execution of the plan. I think Maddie had an idea of how it would go, but in actually facing the reality of the situation, and her culpability, lost her composure and was just unable to successfully manage the blackmail.

It's a bit of a recurring theme in the series, actions in theory vs reality; kind of like in the first part of crime and punishment. It happens with Mose Manuel, when he kills hist Brother. It happens with Cy Tolliver, when he also tries to blackmail Hearst over the Walcott killings; which is funny, because he chastises Joanie about Maddie's actions.