I can't really tell if you were ever really friends or if you were more like acquaintances?
I probably would ignore it, if just acquaintances and similar happened to me, but I don't understand why you've blocked her too?
Even if she is now clean and well, if you aren't friends, I mean, it's not world ending if you don't want to rekindle the friendship, unless you used to be close, which it doesn't sound as if you were.
I know a lot of people who used to use recreational drugs, but I have to admit that crack and heroin sound like quite a big deal to me, but I apologise if that is me being ignorant. Chronic use of them especially sounds very worrying.
If it was a close friend, then being around as support would be important, but if you're just some woman she went to school with..?
I know people do get into drugs like crack and heroin and then turn it around, so you never know and I would never be judgy or critical about it, because there but for the grace of God
.. But I also don't know why you should feel obliged to start up a friendship with her when you weren't really friends to begin with, regardless of drugs.
Tbh, I think, if you'd been a bit less vocal about it and maybe just ignored her, your friends wouldn't be making you feel bad. But if you said, in a group situation "Sally got in touch. I don't want her anywhere near my baby, so I blocked her", that's probably why they've said you're being mean.
Normally, when people I don't know well, whose lives have taken a different path to mine, get in touch on SM, I usually just ignore if I don't want to be friends. I don't block them and then announce it to all my friends.