I can't access the link (403), but I've consistently seen it used to describe the state in which you rent the fiber. As in, instead of renting the service of shuttling data (where the provider puts the light on the fiber), you rent the entire fiber in its dark state and put your own light into it (which means you get to choose what optics you use).
Or, to use an analogy: You rent an unfurnished apartment. You're going to put furniture inside, of course, but you're still renting an unfurnished apartment.
Or, to use an analogy: You rent an unfurnished apartment. You're going to put furniture inside, of course, but you're still renting an unfurnished apartment.