blackmail / extortion
A threat to reveal a secret versus the wider crime of obtaining by threat.
Extortion is the wider word: obtaining money or an act by threat. Blackmail is extortion by threat to reveal a secret or to damage a reputation. All blackmail is extortion. Not all extortion is blackmail. Use the wider word when the threat is force, harm, or official power, not exposure.
Examples
UseHe faced blackmail over the letters.
UseThe officer’s demand was extortion.
Avoidcalling a threat of force blackmail