Do you know if the contact has StuffPlug and the "talk offline" option on?
This is what I think about it:
Afaik, when you block someone and open a window, there's a boolean that tells if you're blocking the person or not (thus, it allows or not you to talk to them). StuffPlug puts the value so that it's not true for YOUR client that you're blocking them. This will start a session with the person you opened the conversation (alerting their client, even if you appear offline to them).
I'm not sure if it's like that when you're blocking someone without StuffPlug. If it works also for those who don't have the plugin, it might be the same. But I dunno. This is all I think it does, but I'd like to see what Cookie has to say about it.
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