In the course of investigative journalism, information is uncovered. It's the process. It's also the norm, when that information is uncovered, for it to be made public when it has relevance to the story.
In this case, Trump shared a dumb meme of him beating on CNN, that was made by a 30-something internet troll. This guy freaked and got super excited in r/The_Donald that the "EMPORER" had shared his gif on Twitter.
CNN's investigators did their usual job in a situation where something like this was making huge strides in the news. They dug (not very far) and found out this guy's information, and reached out to him for comment. The guy immediately realized that the veil of anonymity he used was suddenly stripped away and he panicked. He deleted all of his reddit comment history then deleted his account (He had some winners, like how when he travels to the south he takes his gun with him because it's "n****r repellent") and wrote a public apology to try to get ahead of the whole thing.
Now, CNN has every right to publish this person's information. They are party to a story that has gained traction on tons of news networks, regardless of the fact it's aimed their way. What CNN has said, instead, is that instead of going down the normal route of investigative journalism, they've decided this party doesn't need to be made public in light of the fact he's apologized, shown remorse, and has basically done all he can on his side to maintain his anonymity. They're stepping back from the publication process, and in publishing their statement regarding this guy, suggesting in a way that other journalists should do the same. There's nothing that stops someone from the WSJ or Fox News from doing the same rabbit hole research CNN has done to find this guy and publish his identity. CNN further went on to say that, should this guy do something newsworthy in the future, that fact may chance. If he doubled down and decided to become part of the narrative of the story again, they'd publish his information. That's the gist of what they were putting forward.
I don't think it was blackmail in any way shape or form.