Blackmail is the threat of an action _unrelated to an existing right_.
A good case is Autumn Jackson: https://en.wikipedia.org/wiki/Autumn_Jackson. It was not blackmail for her mother to request $2k/month in child support in exchange for silence, because she had a right to child support. But it was illegal for her to request $40M, because that was unrelated to child support.
Another example: you can threaten a restaurant with a bad review unless you receive a refund for a bad meal. You can't threaten a restaurant with a bad review unless they give you $1K.
It seems weird that a demand for money per month for as long as their was under 18 could potentially be blackmail if the one the demand is made of is too poor. What if someone demands $2k/month child support for someone who, per the formula the court would currently use for child support, only owes $200 a month. Is asking for 10 times the required child support considered unrelated? Going to the other example, that would be like asking for $1K when your bill (and thus refund) was $100.
I think this is just another case of what law really comes down to for most people is how good a lawyer you can afford. The more expensive lawyers generally can make better arguments and get you out of more trouble (or at least get a much lighter sentence). Also, there is a difference between what is legally blackmail (depends upon location), what is morally blackmail (depends upon moral system), and what we linguistically call blackmail (probably more complicated than the previous two combined).
That said most of the continuous space is not close here.
There's broad agreement that people can threaten to leave a room if someone continues shouting at them. But that people shouldn't be allowed to threaten to post explicit photos of someone if they refuse to sleep with them.