I don't want her to KNOW I've blocked her.
Why not? Honestly, it would be better if you did. You can frame it in the positive - "mom, I won't get any texts or emails or calls you make to me. But, we are going to talk on the phone once a week, and (siblings) know how to get in touch with me in case there's an emergency. I want you to know this so you don't get frustrated if you try and get in touch with me; I won't get the message. I'll talk to you (date, time)."
And, block.
It's almost like pretending you are in a foreign country with 1990 technology.
I would fear that if you don't tell her, it is going to consume the relationship, her frustration at your non-response. This way, she can work on her behavior also.
When it becomes the topic in your first call, you just need to repeat: "This is how it's going to work for right now" And "Let's talk about something else. [subject change]"
If she starts repeating "but why are you doing this?" You need to stick to: "I've said that it's what I need to do, and there's nothing more to say"..."I'm sorry, but there's nothing more to say about it"..."I'm not saying any more about it"...."I will not say any more about it"...."no"..."If this is all you want to talk about, let's finish this call and try again next week. I love you, goodbye". And, block.