Okay, I finally found the answer! It comes from https://github.com/getgrav/grav-plugin-admin/pull/1364:
With this patch user can override in blueprint the taxonomy field to add custom taxonomy values in default and options attributes.
There is only one catch - when overriding default taxonomy field, with both default and options attributes, you need to add validation attribute with type: commalist, as shown in example below. Otherwise, after first save Grav will throw an error at top of the page Array to string conversion, and page won’t be saved. But after second try, it will be successful. I haven’t found why this happens, but adding validation fixes it. I’m guessing that it might be related to #1024.
I tried it, and it worked! The only issue I had was with replacing the default value when extending the blueprint. I have two types of pages that I want end users to be able to create, with the difference being what the default category is set to. So, I created a blueprint for a page with the first default category, and then I just extended it and attempted to replace the default category for the second page. I tried various methods, but none worked. I kept ending up with the original default category and my new one. Finally, I just had to replace the entire header.taxonomy, and then I could set the default value properly. Here’s how I did that:
category: ['Saturday Morning Conversation']
I hope this solves your issue.