Good question. I guess it would depend on the damage done, and your state of residence (no fault states may see some of the evidence differently). From your profile it seems like there's a lot of damage to your kids, which is always actionable I think.
Isn't the difference intent to defraud (blackmail) versus keeping the upper hand as the honest party in fairly negotiating with a known dishonest party (leverage)?
I would think leverage is him knowing you have a lot of information that is damaging, so they're open to negotiation so you don't reveal it - the info is a trump-card to keep WS honest.. Importantly, it's cool if they don't know but imagine how much/how deep info you have, as they did a lot of crap. I'd say blackmail is more "do specifically X or I'll show Y info" - like a tit-for-tat.
So in a sense, blackmail is a fraudulent, transactional portion. Leverage is looking at the whole picture.
In my case:
I 'm as strict NC as I can get away with, considering we have young kids. I can't avoid briefly seeing STBXWW today because of a kid switchover at a clinic, but last I saw her physically was a month ago.
I collect what I find. Then I correlate dates with events - what I have on STBXWW, what I was doing the same date, how the kids were doing with me at the time. I keep a spreadsheet. I make notes of possible damage to my kids (no fault state, so the separation of assets is straightforward, but I am very much the stable parent and all of this is to prove she's not during the custody hearings).
Then I send to my Lawyer. He'll know which to use as leverage and what not to worry about. I believe my L may be sharing some of it with WW L, but I don't know, I haven't asked. I think if it goes to trial the Ls have to share everything they will use (not sure but I think that's it).
Haven't had my first custody hearing yet, so I don't know what the guidelines are like (on the orientation they said to try to keep it about the kids, hence the spreadsheet correlation). But it's good that it's providing me a timeline that is verifiable and can be used in court should this go to trial.
I don't plan on making any of this public unless things go horribly, horribly wrong and she decides to invent and accuse me publicly of something or other (which she would need to do in order to turn the tables on this very clear legal case, so she may try in the height of narcissism). Even so, I don't know if I'd want to do it until after D is final - don't want her to use it as an excuse to drag things legally just out of spite.
Still, once she sees that I have detailed records during custody and I'm just sharing what's relevant to the kids she'll wonder what else I have. And since I won't talk to her about anything, that's leverage right there. You don't have to say what you have as long as WS knows it's plenty. They know what they did and seeing you organized and showing only enough for the part of the process you are looking at should scare them enough.