TL;DNR: Blackmail seems straightforward, but it's not. It depends so much on the specific facts of the case that we need an expert to find out how this case would turn out.
A somewhat longer take:
Blackmail is a funny thing. It seems simple and straightforward until you start thinking about it. Take the classic blackmail scheme -- the cheating husband. The blackmailer does two things She: 1) asks the husband for money; and 2) threatens to tell his wife he's cheating. Both 1 & 2 are legal if done separately, but when done together they are a crime. Why is that?
There are lots of explanations for when Legal + Legal = Illegal. Unfortunately, none of the explanations seem to explain the law very well. For example, most of us think it would be legal to say to a thief, "Give me back my property, and I won't call the police," or for a judge to say, "Stay out of trouble for the next year, or I will put you back in jail." But both of those meet the definition of blackmail (as gnasher pointed out above!). So why are they ok?
Which category the CNN/Redditor case fits into depends on how courts have drawn the boundaries between blackmail and hard-bargaining (or other lawful actions). In other words, to answer your question, we probably need to hear from someone who actually knows the law. We also need to know a bit more about the facts of the case. For example, it might matter whether the Redditor offered to take stuff down and behave, or whether CNN offered to keep his name private if he stopped posting stuff they don't like.
It's clear CNN did not help themselves in the "court of public opinion" by the way they explained themselves. Whether they did not help themselves in real courts is another matter.
Eugene Volokh had a nice piece on the complexities of blackmail on the Volokh Conspiracy this week. It's worth a read: