A telltale sign that you may be on an employment blacklist is when potential employers suddenly lose interest in hiring you, after expressing a desire to hire you. While there is no formal way of checking to see if you are blacklisted, you can check your state laws on blacklisting, and then consider taking legal action against anyone stopping you from obtaining gainful employment.
Each state has its own laws pertaining to the practice of blacklisting employees. Idaho, for example, has a law that makes it illegal for employers to maintain a blacklist, and former employers cannot contact other employers to inform them that an employee is blacklisted. In Massachusetts, it's illegal to intimidate or use forceful measures to prevent someone from getting a job. California law states that employers cannot use misrepresentation to block employees from seeking employment elsewhere.
Even though blacklisting is illegal, it can still happen. Some things that can land you on a blacklist include having a bad interview, submitting too many applications, failing a screening test and failing to disclose important information. Creating a bad impression during an interview by saying the wrong thing or acting unprofessional is one way to get blacklisted. Bombarding a company with several applications comes across as desperate and sometimes leads to blacklisting.