My worry is that if I don't respond she won't get her "release" of emotions and she will take it out on my son. Does anyone have experience with this? What happens to the rage if it goes unacknowledged?
Good for you for setting your boundaries and sticking to them! It's unfortunately impossible to know what actually set her off, so it's equally as impossible to know what will happen next. I'm the step mom of two kids with a BPD mom. In my case, not reacting to rages goes one of two ways: 1) It's something that's actually bothering her or is somehow related to something that is actually bothering her and so it'll just get brought up again. Usually in an equally nasty way. Or 2) The blow up has nothing to actually do with us (since there is so little communication this is the more likely scenario in my case) and she just pulled a bunch of old rage out of where ever inside herself and starts flinging accusations. Fight with a boyfriend? Then my DH is a useless uninvolved father. Problems at work? Then my DH doesn't send child support quick enough and should know the children need things. Nothing has anything to do with the other, but she needs to take her feelings out on someone.
If you are unavailable then it'll be someone else. That means family members, children, and friends. But at least in my case, I don't think that my DH making himself an available target has ever helped the kids. She will take things out on them if they are available. But by not engaging he saves himself a lot of grief and the kids might hear "Your father doesn't even care." Which is something they know isn't true. But what they don't hear is, "Your father said awful things to me that I'm going to tell my lawyer!" Which we know is something they've had to hear when she's managed to force confrontations.