I agree that Blacklisting is a horrendous practice. Having worked for some fairly large construction companies n my time, I have known people both responsible for it and affected by it. I would be very surprised though if union members weren't allowed on site. I am pretty sure also that there will be known union representation on site.
I have heard and it is only second hand, that a request has gone in to LoR for specific Union accommodation and site offices for them, which has been refused.
We must also be sure to separate blacklisting of Union members, with the non employment of disruptive individuals who happen to be part of a union.
I don't know which is happening here and make no judgment without the facts, but of it is blacklisting as it used to be, then LoR have some VERY big fines coming their way.