The Live Rock rubble is there for its biological filtering capabilities. It does not require light for this function.
There are some things that can survive on LR which is why we light it in the aquarium. This does not matter if just being used as filtration.
There are also certain filterfeeding organisms that do not survive well in an environment with light. They will tend to congregate in the canister and filter water themselves. Things like sponges and tunicates love dark areas and will really help your tank by eating organic particulate before it can decompose to nitrate and phosphate.