What is the black or dark red on tuna and swordfish and is it okay to eat?
That dark, nearly black area in the middle of your tuna or swordfish steak is nothing bad or unhealthy, although you may not like its strong flavor. It is a muscle that is rich in myoglobin, a blood pigment. But lest that sound creepy to you, bear in mind that myoglobin is the same iron-containing pigment that makes red meat red.
You can leave it in when you cook the fish: the stronger flavor of that one area will not affect the rest of the fish.
Recipe: Pan-Grilled Thai Tuna Salad