Take a Deep Breath and Accept the Blame
You’re not the only one who realizes that the boss is a narcissist. Chances are nearly everyone in your office knows or has some idea. Therefore, when you take the blame, they know that you’re not really to blame. They probably know who is but also realize it’s best to let the supervisor say what they say.
When these moments come, just take a deep breath and let them come. You don’t need to agree or apologize unless you know it’ll cool them down. When it’s all over, take a moment to collect yourself and then spend extra time to take care of your mental health by doing something you love or just be alone.