The old doco at https://eeyo.io/adv/docs/ makes many references to darkness in the context of screen colours. It also makes reference to zones, but this isn't mentioned in the Reference Guide. I was thinking of having a dark zone and allocating dark rooms to the dark zone. Then I can test if a room is in the dark zone. However, I don't know if this is still supported or how exactly it works.
If you have to do it manually, you need to somehow allocate a property to a room to indicate that it's dark by default. Then in the room's on_describe handler, test if the player is carrying a light source that is lit. If they are, give the normal description and graphic. If they're not, give a message saying "It's too dark to see." and a blank graphic. There are other issues to handle regarding movement in the dark, picking up objects in the dark and so on, but let's see what Chris has to say. In the meantime, I'll check out the Cloak of Darkness code.