What to do with threats of blacklisting?

You went to the administrative person. That was your error and the core issue.

Allow me to give an example from the other side in which this happened:

While I was at another institute, I was asked to take over a lab for another person. The lab hosted a guest cohort from another country so the university needed someone with content knowledge and dual language fluency on short notice. So I was asked if I could take over the lab for a week.

Suffice to say, I did not know that there were particular rules for this lab. Some were obvious (like not leaving equipment plugged in) but I was a little overwhelmed with taking over a course last minute. Further, my work is primarily computational rather than lab focused so the reflex nature of lab rules was not there for me.

After a few nights, the lab manager came in and saw the lab after I had left it. Equipment was left out and plugged in. All a very big mistake on my part. The way she responded was where things got messy.

In hindsight, a more measured response would have been talking to me or even the lab's PI to find out what was going; even going to the department head. Instead she went to the Dean. She did not know the situation with the lab and the dean didnt, but as soon as it went to the dean, the situation became "administrative". Funds were threatened and it ended with me having to write a formal apology to the department head and the lab manager with everyone CCed on the email. Everyone was aware the situation and most parties were mainly pleased that I was willing to write the apology so the "administrative" side of the situation could go away and we could fix things in house. (I ended up just getting an assistant who was a little more lab savvy). The department head came by and gave me a sympathetic pat on the back.

Was I wrong, certainly. But it was likely an issue that could have been dealt with "in house".

So, what was the end result of going to the administration rather than keeping things in house? At one end, awkward elevator rides when the lab manager and I crossed paths. She found out the situation, and to find out, she didnt even know I was in the lab. I heard she thought it was an undergraduate group. On the other end, it did leave some lasting trust issues and undermined what had been before a very good working relationship between two labs. But I think that things have smoothed out since that happened.

In other words, once you go to the administration, the situation becomes administrative.

You have not quite told us what happened with you, your advisor and your research group. But even your vague description betrays what I believe to be a lack of awareness to certain aspects of professional/interpersonal interaction which is now biting you in the ass.

This was initially a fairly minor thing. I just needed to explain to others that there was some confusion which my advisor admitted to me privately

This was not a minor, thing - it was a major thing. A huge thing probably. Why?

• You repeated something told to you in confidence, to outsiders, and made it (somewhat) generally known.
• You assigned blame to your advisor while (seemingly) hinting he was incapable/unwilling to take on the blame, thus slighting his character as a person and as a manager.

... and this was even before any official complaints were lodged about anything and anyone.

Now, maybe that's not the only way to look at what happened, but it could very well be the way that your advisor and many others - in and out of your research group - see things. It's possible that, on the merit of the original matter, what you said was true - but that doesn't help with how you conducted yourself.

This whole situation has made me disgusted.

You must realize that the feeling may be mutual. Try to look at things from the other person's perspective - what do they expect and believe - and be aware that they will judge you from that perspective, not on the basis of the facts, or what you consider to be the facts and circumstances.

What on earth can I do?

Unless it jeopardizes the integrity of yours and others research - swallow your pride/disgust, stash your ego somewhat, and apologize to your advisor for your conduct. Be, or pretend to be, penitent.

Does this mean that he's not really to blame for anything? Not really. Maybe most of the blame is with him. But - that doesn't really matter. You have to be generous with taking blame and cutting people slack (whether they deserve it in your opinion or not), as long as others/society/science don't suffer as a consequence. That's an important salve to apply to situations of people being offended.