When I was looking at Philippine English I encountered the wet kitchen, the kitchen in which food was actually cooked by servants. The kitchen in which the hostess made tea and coffee and handed out dainty morsels was the show kitchen of the house. The wet kitchen was tucked out of sight.
The dark kitchen is similar in that it is not a kitchen that serves a restaurant in which diners come to eat. There are no customers, just delivery couriers taking meals to people who have ordered by app. The kitchen staff may be from a number of restaurants who share the premises to cut costs. Most people ordering food online from a restaurant assume it is coming from the place they know and love, but it is more than likely sizzling on the stove of a dark kitchen.