Usually, dark fiber means that you got that cable (with one or more hairs) completely for yourself, as a customer. It is not leased line, you are not sharing it, it is your fibre to communicate with. It is more expensive, for sure, since Carrier could make lot of $$ out of it (many many Internet Customers for example), but at the end of the day it is cost efficient actually since you have no other interference on your cabling. You do not share bandwith, you do not share services, you decide about your techonlogy at the endpoints (GBICs, etc.).