Also, the term looks like it has both meanings.

Having a "dark fiber infrastructure" means having a web of unconnected fiber cables point to point, but since you connect your own devices to it, you don't have to share it with others.

As a result, its presence and its state is only known to you (i.e. it's in the dark).

I understand that my usage is not completely true, but in that context, it's not a flat-out blatant mistake either.