The act being threatened – reporting your crime to the police – is totally legal, even socially encouraged. It is only the demanding of money (or other benefits) not to do it part which is the crime of blackmail.
If I just went ahead and reported your crime to the police – no crime of blackmail.
If I just didn't – no blackmail (but could be some other crime, such as misprision)
It's only when I tell you that whether I'm going to do it depends on whether you do something for me that blackmail has been committed.
Now a more relevant example:
I discovered a security vulnerability in FB.
1. Publish the vulnerability publicly (legal)
2. Sell the vulnerability to exploiters (illegal)
3. Accept FB's bug bounty reward (legal)
4. Attempt to negotiate a different amount with FB, falling back to #1 (illegal, blackmail)
5. Attempt to negotiate a different amount with FB, falling back to #2 (illegal, extortion)
6. Attempt to negotiate a different amount with FB, falling back to #3 (legal)
I assert that treating #4 as a crime is to use the police powers of the state to protect FB's wallet.
But I also think a lot of these companies are happy to frame negotiations as extortionate if someone has the audacity to counter their offer, and that's bullshit. But it would also be bullshit to try to drive up the price on a real bounty after the fact by threating to let the bountied person run free with a map to your house, so it's complicated and I can see the precedent in thought there.
The classic situation of blackmail is demanding money from someone, or else you'll reveal some embarrassing fact about them, report that they committed some crime, etc.
Saying "Give me money or I will publicly disclose a bug in your computer systems" – that fits the classic situation of blackmail straight on.
Saying "Fix this bug in 90 days or I'll publicly reveal it" – doesn't fit the classic situation of blackmail, no demand for money involved.
Now, not all cases of blackmail fit the classic situation. It is possible for a person to commit blackmail without demanding money, if instead they demand something else of direct value to them – for example, saying to a university president "Offer my child a place or else I'll tell the media that you are cheating on your wife".
But, in the case of Google Project Zero style "Fix this bug in 90 days or I'll publicly reveal it", it isn't clear that the demander is actually demanding anything of any direct personal benefit to themselves. Generally speaking, the direct personal benefit to the security researcher of the bug being fixed is going to be negligible. If I demand you do something which doesn't directly benefit me (or my family or friends) in any tangible way, I don't see how such a demand could legally count as blackmail.
I doubt the delay itself has any direct legal relevance. Going to someone and saying "I'm going to report your crimes to the authorities no matter what, but if you don't pay me I'll do it tomorrow, if you pay me I'll wait until next week instead" is probably still blackmail. (Getting one week's notice is invaluable if you plan to flee the country, for example.)