Blacklisting of employees is misdemeanor in most states (fines of up to $500), and also permits employee to sue for damages.
Example state law (pretty much the same in all states):
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Employers have limited immunity for good-faith provision of employment references upon request of current or former employee. No immunity if references are provided without authorization from employee. However, standard rules applicable to defamation claims continue to apply where references are provided without express consent of the employee (so the immunity provisions of the reference statute do not appear to provide much , if any, additional protection to employers).
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In the real world, few companies are ever prosecuted for giving negative references to employers. What happens most often are lawsuits against former employers.
Leery of lawsuits, an increasing number of employers are avoiding employment inquiries - or opting to "No Comment" when asked crucial job performance questions. Even if the performance of the past employee was excellent. This is a gross disservice to all employees who dedicate themselves to a company and naively expect to be compensated with a favorable recommendation - yet in truth they receive no better of a reference then an employee who was fired for misconduct.