Canadian law defines a crime of extortion, but not blackmail. An alternative theory of "blackmail" is that it is coercion. This is the law against extortion in Canada. We start with the definition of extortion, §346 (1)
Every one commits extortion who, without reasonable justification or
excuse and with intent to obtain anything, by threats, accusations,
menaces or violence induces or attempts to induce any person, whether
or not he is the person threatened, accused or menaced or to whom
violence is shown, to do anything or cause anything to be done.
To simplify, one person makes another person do something using threats, accusations or violence and doesn't have a good reason to do so. A parent can altruistically threaten to write a child out of their will if the child doesn't stop taking heroin: that's a good reason, so that is not extortion. If you threaten to turn a murderer in to the police unless they give to $1,000, that is extortion. However, the law also provides that "A threat to institute civil proceedings is not a threat for the purposes of this section". It is legal to threaten to sue the pants off of a person if they don't settle for some cash reward.
The wording of the law is very broad and it's not obvious from the wording of the statute where the line is drawn. These guys give a number of examples of acts leading to convictions:
A teenager who said he would bring an AK-47 to school and shooting
people unless he was compensated for his damaged vape;
A man who was found to have implied violence by asking for money and
stating that ‘things will get a lot worse’, and ‘if you can’t pay with
money you’ll have to pay with something else’ if he didn’t receive it;
Three men who were accused of threatening, harassing, intimidating,
and extorting a man who tried to start a new chapter of a
Montreal-based Motorcycle club;
A man who allegedly committed acts including arson, telephone threats,
molotov cocktails, and paintball-gun shootings against his former
business associates;
A man who threatened to post nude pictures of people on social media
unless paid;
A man who pretended to be a photographer and threatened to send nude
pictures to family or pornorgraphic magazines unless sexual favours
were granted;
A person who threatened to sell a story to a newspaper unless the
victim paid a sum of money (the story regarded the victim’s recent
conviction);
A woman who threatened to send a letter to another woman’s employer
unless the other woman repaid a debt she owed
As they say, "the threat must go beyond what a reasonable person in the accused’s situation would view as a legitimate means": the issue is dealt with on a case-by-case basis.
In case you murder someone or commit tax fraud, and another person threatens to turn you in if you don't {pay them / mow their lawn}, the law takes the position that you should turn yourself in, provide the evidence to the police, and the police will prosecute the other person for extortion. There is no "I was blackmailed" get out of jail defense.
One practical implication of the law is that you can't extort a person because they committed a crime: instead, you are expected to just turn them (and the evidence) in to the police – that's your civic duty. You can also hold your tongue: what you can't do is extort the other criminal.
