These terms, while used simultaneously, are actually somewhat different. Extortion involves obtaining something from another party through various underhanded methods, such as intimidation, threats, or violence. An example of extortion is a character threatening to reveal a neighbor's extramarital affair unless the neighbor provides use of his pick-up truck (possibly so the character can bury a body or transport something without using their own vehicle). Blackmail is the act of using extortion to specifically get money from a person. The preceding example would be considered blackmail if the character demanded $10,000 instead of the neighbor's truck. Anyone can employ these practices, but a character with ties to a gang or the mob will be especially familiar with them.

Dark or Disturbing (primary), Betrayals and Violation of Trust

BEING BLACKMAILED, GAINING WEALTH THROUGH UNETHICAL MEANS, WEAPONIZING OTHER PEOPLE'S SECRETS

BEING ATTACKED, BEING HUNTED OR PURSUED, BEING WATCHED, DEATH, GOVERNMENT, INADEQUACY, LOSING AUTONOMY, LOSING FINANCIAL SECURITY, NOT BEING IN CONTROL

Feeling unable to go back to being who they were before the extortion
Regretting their association with a gang or criminal organization but feeling unable to escape it
Worrying that the other party will choose to not comply, revealing the character's crime
Feeling watched or followed when they have dealings with the victim
Reluctance to reveal the other party's secret; desperately hoping they comply with the demands so the character won't have to take further action
Having to keep their “reward” secret (not being able to spend the money publicly, having to hide whatever possession they extorted, etc.)

Keeping distance from the victim; not associating with them 
Pretending to be good friends with the victim; staying close to them to keep the threat real
Coming up with a legitimate reason for spending time with the victim
Disabling security cameras near the victim's home or business