What you describe is indeed blackmail.
"If you do/don't do <thing>, I will <stuff>" is the blackmail formula.
I'm not an expert on blackmail, but I'm pretty sure the above in certain contexts could be considered a warning instead of blackmail. If a police officer tells you to stop speeding, or else you're going to suffer the consequences, that's a warning. He isn't looking to get something out of the transaction personally--and I think that's a key component of blackmail/extortion.
If someone sends a PM to a member on bitcointalk stating that if they don't stop spamming in a thread that they're going to get reported/negged, I would see that more as a chance to correct behavior before the offending party suffers some consequences. It isn't threatening to expose someone in exchange for money/favors/whatever, which is usually what blackmail is.
blackmail is when you are asking for something, usually money, that they do not have a legitimate claim to/that they are not entitled to.
For example:
Asking someone to pay a valid debt owed and saying nonpayment will result in a lawsuit for the debt is not extortion.
Asking someone to pay $5000 when there is no financial relationship (purchased or otherwise), or else they will call the police on them is extortion because they don’t have any claim to the $5000
You can read
this for more information about extortion.
