The term 'dark' is also used in real estate investing circles. For example, if K-mart or Publix vacates a location before the end of the lease, but continues to pay the lease payments even though they don't continue to operate the store, then the owner/developer has a mall in which the anchor tenant is 'dark'. In this case, the developer sitll receives lease paymetns but this is bad for the in-line tenants since many depend on traffic created by an operating anchor tenant for their business. Sometimes the term is also used if the major tenant simply vacates after their lease is expired. Regardless, in both cases, its bad for the non-anchor tenants if an operating anchor is not present.
So, if you ever open retail shop that is not a destination store (like a dive shop) in a shopping center where you depend on traffic created by the anchor, you'll want to be able to to have a provision in your lease that lets you get out of the lease if the anchor 'goes dark'.
Now you can talk shop with your real estate investing friends....... now back to diving after going down that rabbit hole