You are looking for the right tool--boundary enforcement.
I'd like to suggest you use it a bit differently. It works best for me if I state boundaries (to myself) this way:
"When you do 'X', I will do 'Y' to protect myself from the consequences of 'X'"
Not "When you do 'X', I will punish you by doing 'Y'." This variation is being controlling, and will piss off anybody, pwBPD or not.
Not
"You cannot do 'X'." This is a rule, and when you make a rule, you are challenging her to break the rule, which she will do. The choice and power are all hers this way. Look back at the first one, and note that you have the choice and power, as you are choosing to protect yourself.
One important note--You can choose if enforcing the boundary includes telling her you will do it either before or as you do it or not. For some boundaries it is the right thing to do. For others, it only escalates things and makes it worse.
OK, now applying better boundaries and enforcement to your situation:
Mentioned to GF this am that I was seeing [religious counselor], and here comes the insults.
I'd start with this thought: "My counseling or therapy is my own business and not something I'm discussing with you."
Since you are in a relationship with her, she has a right to know about your schedule, when you are available and when you aren't. In your shoes, I would avoid telling her that you are busy at a counseling appointment if you can do it gracefully, but not lie about it. If she's asking/interrogating around it, I wouldn't be evasive. I just would choose not to bring up the topic myself, as I don't borrow trouble.
However I would be firm that what you discuss there is between you, the counselor, and God, and not her business. The active boundary enforcement would look like:
"I will not discuss my counseling with you. If you ask, I'll tell you, and if you won't let it go, I will end our conversation."
You see how all your actions are specifically designed to protect you from being interrogated about your counseling session, and don't do anything more? You aren't punishing her for asking about it.
So she leaves. Then the phone blowing up starts. Cell phone, house phone, texts, messenger messages.
You had the right idea here, but it can be improved. What you want to do is immediately protect yourself from the phone blowing up. No more, no less.
Seeing all those messages, hearing those phone rings is stressful to you at that time. Protect yourself. Preferably with the least force you need for the job.
Can you "mute" her on the cell phone and on all the messaging/social media platforms, so you don't get notifications for the dozen attempts to get your goat there? If you can't, then block her. You can do this each time she gets to a new medium.
I sent her back a text and said there is no point in talking right now, you will be ugly and run me down, and it will piss me off and I will be ugly back. So I said my new boundary with you is when this happens, I am not talking to you for 24 hours and until you apologize.
Saying that you aren't talking to her now is excellent. Admitting that you will be ugly is fine.
Don't make it about her. "... .you will be ugly... ." You know how likely that is, but telling her what you know she will do is invalidating and will piss her off. Don't do it.
"I won't talk to you for 24 hours" sounds like punishing her, or a threat. And it isn't your purpose--you just want to get away from her blowing up the phone, and her verbally abusing you. If it stops in an hour, and if you calm down by then, that would be fine. If it lasts for a week, you aren't ready to dive back in for six days tomorrow!
"until you apologize" isn't going to help you much either. If she's just saying the words to get back, it won't mean much... .and if it is sincere, it will be offered without the threat/demand.
Better is to make it about yourself. Try this:
"I'm too upset to talk to you." Not at all about her, and completely true as well!
How much time do YOU need to calm down and center yourself? You can tell her something about this too, and if you do it right, it helps--shutting her out triggers her fear of abandonment, and you ARE shutting her out for your own protection, but you can be clear that it isn't forever, which will help.
Don't use words like "soon" or "later" or "a while". Those are vague, and risk sparking a fight when you come back about what it meant and whether you lied about it. Something like "I'll check in in an hour." or "I'm going away for two hours" or "I'll talk to you after work (assuming your quitting time is consistent)" is safe.
And note--if you promise to get back in touch, that is exactly what you promised--you are NOT promising to accept a bunch of abuse--If she launches back into you when you reach out, remove yourself again, immediately. (And likely you should take more time away than you did last round!)