I think it's really excellent for people with BPD if they can do it safely. I also used to journal a lot. Writing sounds like it would really help in your situation. It's important to exercise your communication skills when you're isolated and keep them from degrading. It also helps you process your emotions (and gain some distance/perspective) through expression and self-reflection. Also, having old journal entries can be really helpful for understanding yourself and how you change and don't change over time, something those of us with BPD tend to particularly struggle with (but all people lack self-awareness in).
I don't currently because I have severe motivational issues with writing. I have memory loss issues and my previous journalling is invaluable to me, so it bothers me a lot. I used to keep a digital diary (through LiveJournal, so social blogging isn't the same thing, but I seriously wrote quite a lot and very regularly), and before that, I kept physical diaries in whatever was handy (and I still have them, but I was very afraid they would be read also).
If you would prefer to physically write your diary but still worry about it being found one day, you could make a habit of digitising your entries (you could even use your phone as a scanner) and destroying the physical copies (or destroying them in general, if preserving them is unimportant to you, but I don't recommend it because the perspective given by a diary is so helpful when you have feelings that fluctuate...). Data can be easier to secure in a lot of situations. It might be easier to habit-build around using a physical diary in a specific setting, and might be helpful detoxing time.
