In my experience I've found that every industry is its own little community. This is much like every "interest group" is its own community. Reddit, or HN, are communities.

The tech community has fairly high mobility; people move from job to job through out their career. Often times they will segregate into an even more exclusive sub-community be it 'networking person' or 'security person' or 'language person' etc.

Companies spend a lot of time and money recruiting, training, and evaluating their engineers. These engineers are the "secret" to the success or failure of a company's efforts to execute against their vision and plans so companies spend time and effort to understanding as much as they can about engineers. And when they can't understand enough they count on 'sourcers' or 'recruiters' to understand as much as possible about the engineers.

As I have worked at companies, both large and small, and built up companies from nothing to something. Every single one has encouraged employees to refer their friends knowing that good engineers know other good engineers, and they know bad engineers as well.

As a result of these forces, what emerges for any given industry, is a sort of ranking and rating system for pretty much every participant. There may be six degrees of freedom between any two people in the world, but between two engineers in the same specialty? It often seems it is often 2 or at most 3 degrees. As a result there is a giant sort of collective knowledge about people in the space after they have been in the space for a few years.

Now consider what a company is going through when hiring. The manager may have had to argue with their management to get a hiring requisition 'approved'. They probably interviewed many candidates and had group meetings with the interviewers over plusses and minuses, skills and deficits. They winnowed that set of candidates down to a preferred choice and made them an offer. Which the candidate accepted. It's a successful process and probably the manger tells the team the good news and everyone is looking forward to having some additional help with the project. If the recruiter/hiring person is on contract they are excited because they are looking at anywhere from 10 to 25% of this new hires annual salary as compensation for getting them hired. Perhaps one of the employees in the group made a referral and they are excited that they might be getting a referral bonus.

Now the new employee calls them up and says "Sorry I got a better deal elsewhere so I won't be joining you after all."

The expectations are dashed, the hiring process has to be reset to the initial square, perhaps a strong alternate choice has been missed because when they didn't get the offer they accepted an offer somewhere else. And the recruiter isn't getting their big payday after all.

This lays down a lot more 'hurt' than young engineers can really appreciate early on. And that 'hurt' can, and will, affect their future chances. Is it black balling? Yes? No? Maybe? It is a natural consequence of people being let down they lose trust. The manager, the recruiter, the other members of the team that interviewed this person, all of them now are a walking and talking 'negative signal.' And they will be for possibly several years to come. And since the community is generally more interconnected than you might imagine it is, the chance that at your next job change this event will come out and have an effect. A good team would bring it up and ask about it, but many teams (especially ones where there are many choices) might just move on to a different candidate.