You can make a complaint to the State, and the accountant may lose his/her professional license. Most accountants will jump through a lot of hoops to avoid or resolve a complaint to the Board.
Here is your info: http://www.mass.gov/ocabr/licensee/dpl-boards/pa/faqs.html
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That's entirely illegal. You should consult an attorney and should seek advice.
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Echoing the previous answer, a threat to disclose confidential information that doesn't belong to the person threatening to disclose may be "actionable", and if the person threatening to disclose is prohibited from doing so (normally) by professional rules and or laws, they may also lose their license. However, the character of the information can be relevant - i.e. if someone was doing something improper and harming the public, and the blackmail was about that, the rules are a little different, but blackmail can still be actionable. You should contact a lawyer.