I'm so sorry that you're going through this! It sounds like you smartly removed yourself from a bad employment situation - and good for you. This is a dificult choice to make, and comes with lots of mixed emotions. You'll probably doubt your action for a while. But I'm sure you gave a lot of thought to this decision, and didn't just huff out the door spontaneously one day. It's like dumping a bad boy/girlfriend - you love them and might miss them, but you decided over time that the relationship was bad enough to call it all off. It will take a while to get over. Let yourself be sad/bored/angry, but also celebrate your liberation. You were the smart mouse that got off the ship before it sank.
Now, about this lousy boss... First, I would wonder what kind of conversations are actually happening in which she is blacklisting you to other companies. It is extremely unusual for Big Cheese to call up Impressive Boss and say "hey, by the way, this Stage Manager is a horrible person and bad worker and you should never hire them," and Impressive Boss says, "oh, gosh, OK." Usually, when someone tries to speak against another professional, it reflects badly on the speaker and really doesn't affect the reputation of the worker in question at all. I know, opera is a small field in which everybody seems to know everybody. But it is also filled with plenty of executive drama and staff turnover turmoil. This too shall pass. Keep a high profile so people can see you are not ashamed and not afraid of the rumours being spread by Nasty Boss. Keep going after jobs, and take a small show or anything where you can show off your mad SM skills to remind yourself that you rock at what you do. Are you considering moving/travelling/touring for more opera SM gigs? If the boss is making it really hard to continue in your town, you truly might need to move to nurture your career in a different climate.
You'll get through this, and they'll be sorry to have lost you. I promise!