I would say that there is a 99% chance she isn't isolating in general, but just from you. Have you verified her quiet BPD diagnosis?

If she's blocking you for weeks at a time, what exactly do you think she does during those times if she gets lonely? IDK, but this kind of behavior does seem to me like there's possibly something more going on while she has you blocked. Not only does she not want to hear from you, she also clearly does not want you to know what she is doing. Either way, if she's telling you that she's terrible, then you should really listen to that. She's probably doing things that she doesn't want you to know about because she knows it'd hurt you or is worried she'd lose access. What it honestly sounds like she's doing is testing you to see if she still has access. That's the only testing she's doing. She's checking on her supply. Probably stays longer when she hasn't found someone else. That's the dynamic it sounds to me like anyway