Dark fiber is a fairly well-accepted industry term for fiber that you operate yourself, with your own lasers, as opposed to capacity that you lease from a telco and just provide your bits via serial or whatever.
It has also been considered to be pretty secure against eavesdropping due to multiplexing and just the sheer amount of data that can be shoved down a single long-haul fiber these days (5+ Tb/sec). Unfortunately, that's no longer necessarily true when you're up against an adversary with nation-state resources.
