And as always, even worse-- You won't ever know if you just aren't up to snuff as an engineer / interviewer or if one or more of your interviewers, recruiters or hiring agents knows of and/or participates in such "blacklisting"

Which is also illegal, but companies openly get away with it every day.

The worst that ever happens is a slap on the wrist when all the major companies have non recruitment ("no poaching") agreements with each other, suppress wages, get caught, and have to pay a token fine.