So I've wrapped my head around blacklists & tuning files - but despite reading the documentation multiple times, I still can't figure out how you import your blacklist to a new save.

What I tried:
In game: Fresh game > Choose sim > Open / close CAS > Save & close
In Packer: Open the .package > Double click > Open .xml blacklist > Save

Creating-Invalid-Parts-Tuning-the-Easy-Way

You don't need to import it, the invalid tuning package file in your mods folder is enough and will blacklist the items for you automatically.

But you won't see the items in MC blacklist in new saves (the ones where you didn't blacklist in-game).

To clarify, so you're saying even though in CAS it says Blacklist: 0 - that it IS indeed working regardless?

Yes, that's how the tuning works. You cancheck yourself, look into your original inthe blacklist in MC where you see the items icons and remember one, also in which age and cas category it is, then see if this item is showing in your other save, if not, everything is right