What is the narrative purpose of the Black Ships in the Traveller mythology? When were they first encountered? Are they supposed to represent an ancient threat, like B5's Shadows? Are they technologically superior to the Third Imperium and thus an invasion risk, like Star Trek's Borg? Are they something you throw at your players when they're "doing it wrong" to discourage a given course of action? Help me understand where they fit into the setting.

Basically the title. I don't see a Traveller group on Eternal September, so I suspect even if it was a thing on Usenet, it's not anymore. I'm not sure how to perform a more thorough check off the top of my head.

I have no idea about Echomail.

If they existed, what were these communities like? Do they still exist and I just didn't find them when I looked?