Not legal for sure. But there are things that are done informally that are similar. Like at one company where I was a hiring manager we used head hunters heavily. Most head hunter companies are bad (in my experience) but the better ones try to match good candidates with good companies. They have extensive networks with other head hunters and often see the same candidates repeatedly. When they see red flags they cull those candidates or maybe present them to the "bad" companies (the ones they aren't concerned about damaging their relationship with).

Another method is that hiring managers at different companies may have good relationships with each other. Not competitor companies, but adjacent. They may call around and ask about a candidate. You can never say anything negative about a candidate; that's just asking to be sued. I think many (most?) companies have a strict policy that you can only say if a candidate is eligible for rehire or not, nothing else. So you talk in vague generalities, never about the candidate, only about the general hiring process. Something like "Company policy prevents me from giving any recommendation whatsoever, so my advice is that you should always be very careful in your hiring process" versus "Company policy prevents me from giving any recommendation whatsoever, but an effective hiring process can yield great employees." You might argue that a judge would see right through that and say it is obvious which candidate you are recommending and which you are panning. But it's so nonspecific that your lawyer can strongly push back, should it come to that.

> Are these legal?

Would that change anything ? Of course those lists wouldn't be broadcasted publicly.

> Does anyone have experiences with blacklists from the hiring side?

There's that "over 40" list I have seen once when I was on the hiring side. There's also that "good gender" list which benefited me when I was on the hired side once.

Not paying the minimum wage is illegal, as well. Doesn't stop employers from cheating ~20% of eligible workers in US on that.

Most likely it’s field-specific. There’s been things like this in the U.K. where construction workers were blacklisted if they joined a union.

Yeah, in my experience with bad workers managers just want to be rid of them. The last thing they want is to be saddled with making sure he is punished after leaving the job. Maybe I just have not experienced the true psychopathic boss yet.

Well you read about them in the Great Depression, the workers who lead the rest to strike would in fact end up on blacklists. Get blackballed, never get a job anywhere no matter how small a wage they were willing to work for (before minimum wage, well irrelevant), they were known by face (photographs, happened to me) but in the employment situation in America today, it's not to that point of there being blacklists, and besides strikes and unions are not the answer they were then.