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