Hi cheeseplease and welcome to the group

It's so tricky when we need to do something for ourselves and our values, and we have a sense that it just isn't going to go over well. As reasonably kind people, we don't want to increase pain or conflict if it's at all avoidable... such a tough needle to thread when BPD is involved.
One thing I have to keep reminding myself of is that BPD stands for Borderline Personality
Disorder -- I cannot assume "oh, pwBPD basically think and process like me, except with a diagnosis". Thinking and relating patterns are
disordered, not rational/functional/predictable. So sometimes we think, "If I just phrased it as ____________, I think she'd feel cared for", or "once I find out the technique for communication, it's guaranteed I can get through to her", or "based on how it seemed like she felt lately, I bet now is a good time for me to say __________". I'm just not sure it's that predictable -- we don't have that level of control over how anyone feels, really, and over how a pwBPD feels, specifically.
That's not to say "do and say whatever, there's nothing you can do". There are some structured approaches to communicating with a pwBPD that can be more effective than "intuitive" ways we communicate.
For example, in terms of feeling a need to make a statement of truth about yourself, the
Support, Empathy, Truth framework can help.
In terms of making a request or trying to problemsolve,
DEARMAN can be effective.
And when stating what will happen, what you will do, or clearing up misunderstandings (especially in written communication), keeping it
Brief, Informative, Friendly, and Firm may be the way to go.
But beneath all those approaches, it's our mindset that matters, especially for a question like this:
How can I tell my friend that I'm moving out without making it seem like I want to end our friendship and without making it seem like it's her fault (even though it is)?
For me, big picture, I might be trying to find a balance between: I'm going to do the best I can to communicate in the most effective way I can learn,
and I don't control my friend's perceptions, beliefs, feelings, or responses,
and the way she responds to my communication isn't what determines whether it was the right thing to do.
Because BPD may be involved, she may not respond in a predictable way to your communication approaches -- I'd say that's a feature, not a bug, of BPD. It may take some sitting with your own discomfort, and some acceptance that you can't control the outcome, to make it through.
Drilling down into details, my gut feeling is to keep any communication about your plans
short. Avoid long explanations, justifications for your choice, trying to "get her to see" your perspective, or giving off a sense of wavering or that she can argue you out of it. Warmth, empathy, and firmness may help:
"Hey, just a heads up that I need to move and my last day in the apartment will be 6/30 (I'll keep paying rent through 8/31). Can't wait to figure out a game night with you so we can stay connected!"
Who knows how she'll respond to that -- it may depend on whatever her inner experience is in that moment -- but returning to short responses full of warmth and desire to connect, may be more effective than
Justifying, Arguing, Defending, or Explaining (or taking bait she may offer):
Her: "I can't believe you'd just leave me like that. I have no idea where you're coming from. Do you ever even think of anyone besides yourself?
"Yeah, it does suck, for sure. I don't love change either. What do you think about Fridays for a day to connect, maybe at wine night?"
...
Curious if any of that seems helpful or on target?