Black cod, Anoplopoma fimbria, is actually not a member of the cod family at all. This fish is known for its blue-black skin and bright white flesh. Unlike many other white fish, its flesh is fairly high in fat -- and this fact makes it difficult to substitute other fish for black cod in recipes.
Black cod is sometimes known as sablefish. It's also sometimes incorrectly labeled as butterfish.
Chilean Sea Bass
The closest equivalent to black cod available is Chilean sea bass. It's similarly fatty and well-suited for dry-heat cooking methods, such as broiling and grilling. Chilean sea bass has a sturdier, less flaky texture than black cod. For several years, Chilean sea bass stocks were overfished and consumers were urged to avoid buying this fish, but in 2013, the Monterey Bay Aquarium declared several sources of this fish either as a "Best Choice" or a "Good Alternative."
Escolar's flesh is white and fatty, making it a good candidate as a black cod substitute. It's not as flavorful as black cod and its texture is firmer, but it can be hard to find in the market.
Escolar is known to sometimes cause gastrointestinal distress and have a laxative effect on some diners, so consume it with caution and in moderation.
Salmon's flavor is unique, but like black cod, this fish is high in fat and buttery in texture, and thus can be used as a substitute in preparations like miso-glazed black cod. Ivory king salmon also reproduces the visuals of black cod's pale flesh and has a similar tender and buttery texture, but it is quite a rare and expensive fish to substitute for the more moderately-priced black cod.
Halibut is firmer and leaner than black cod, but its flesh cooks up moist and flaky, making it a workable substitute for black cod in a wide variety of preparations.
Like black cod, Arctic char is a cold water fish. Its texture is delicate and its fat content is comparable to black cod, although its flesh is darker.