Yes mate, precisely. Some people are disruptive without being part of a Union, they also end up not getting hired.
I know people who have been labour managers for a long time, and have had lots and lots of productive (and lots of unproductive) conversations with Union reps, and in most cases an agreeable compromise is reached. They do know though, through experience, who wont have civil conversations, so they avoid having to have them, by simply not hiring them.
The main thing that was unlawful about the historic black listing was that the companies involved kept a centralised list and actively prevented workers gaining employment with any of their competitors, without any knowledge, reasoning or chance of appeal. It seems the argument now is that they are using agencies to do that by proxy. The allegations are that agencies aren't putting them forward due to feedback at the request of the companies.