What shows the names of people whom employers agree not to hire?

You may be refering to the "blacklist", which if someone's name is on it, they are regarded as "blacklisted". However, it is not a literally existing list, like an actual piece of paper.

It's just that people in a given industry know each other, so if you upset one of them, they mention it to their friends, and when you seek a job with them they remember, and so don't hire you.