What’s that dark part in fish; and is it safe to eat?

If you have ever looked closely at a fish fillet and noticed a dark, almost black strip running through it, you are not alone. Many people wonder if it is safe to eat or if it means the fish has gone bad. The truth is, that black section is a completely natural part of the fish.

That dark portion is called the bloodline or dark muscle. It is the muscle the fish uses for continuous swimming, so it contains more myoglobin, the pigment that gives meat a darker colour. Because it is richer in oxygen and fat, it looks darker and tastes stronger than the rest of the flesh.

Is it safe to eat?
Yes, it is safe to eat. The black part of a fish is not harmful in any way. It only differs in taste and texture. The darker muscle tends to have a stronger flavour that some people describe as fishy or metallic. This happens because it has more iron and fat, which can oxidise faster than the white flesh.

If the fish is fresh, the dark muscle will taste clean and rich. When the fish is old or stored poorly, this part can develop an off smell. That is why chefs often trim it away in delicate dishes like sashimi or lightly cooked fillets. For curries, stews and fish cakes, it can actually add depth and richness.

When you might want to trim it
In large predatory fish such as tuna or swordfish, the bloodline can hold slightly higher levels of fat-soluble contaminants. Most home cooks and fishmongers remove it for a cleaner taste and appearance. For smaller or medium fish like mackerel, sardine or pomfret, it is perfectly fine to leave it in.

According to the US FDA, dark muscle in fish is edible and safe when the fish is handled and stored correctly. Food experts explain that this part of the fish is naturally darker because it is more active muscle tissue, not a sign of spoilage.

Final word
That black strip on your fillet is simply the fish’s hardworking muscle. It is safe to eat, rich in nutrients and only stronger in flavour. Trim it if you prefer a milder taste, or keep it for dishes that welcome a little extra punch. Freshness matters more than colour when it comes to enjoying good fish.