Hi Zippyfox and welcome!

Those splitting episodes, being painted black -- they can be emotionally exhausting and painful to deal with. I'm so sorry. You're definitely not alone because most of us here have been there.
You indicate you've tried not responding. Is there anything else you've tried? When she escalated to telling you never to contact her again, how did you respond? It sounds like right now, her message chain approach is working for her in some way. A key is to find a way so that it doesn't work for her anymore. Looking at your responses and her reactions can help point us in the right direction.
We have an article here on boundaries that I've found very useful:
https://bpdfamily.com/content/setting-boundariesAnd here's a workshop on the same topic:
https://bpdfamily.com/message_board/index.php?topic=61684.0It won't give you an exact blueprint for how to handle this situation, but it does address some fundamental issues and there are some examples.