Whilst I never experienced this myself (although there was about 2 months of frantic attempts to win me back after she discarded me) I think this is a fairly typical case of a recycle attempt.
She knows that you are with someone else and might be moving on so she send a "last" contact and blocks you in the hope that you will chase after her. She almost certainly feels invalidated that you are moving on and is hoping that her "abandoning" you will cause her to chase.
This makes sense to her because that is exactly what she assumes that the fear of abandonment drives everyone the same way it drives pwBPD.
I would take a sigh of relief at the fact she has blocked you and get on with your life including enjoying your new dating/friendship opportunities.
