Legal Question in Employment Law in California

Blacklists are illegal in California (Cal. Labor Code section 1050). But that does not prevent employers from making truthful statements about employees. If you actually find a blacklist, you would have evidence of an unlawful act.

Most efforts to blacklist employees, by ex-employers, are done verbally, which makes it harder to prove. It usually requires the assistance of a prospective employer with enough integrity to be honest about what the ex-employer has said about the employee.

Not that you'll ever find or prove. All the prospective employer has to do is call the old employer to 'confidentially' ask about you. Even if you could prove they said something about you, you would have to prove materially false and slanderous statements were made. Their opinion that you are a lousy employee they didn't like is not 'slander', but opinion.