Start by selecting a firm white fish such as tilapia, cod, or mahi-mahi. These types of fish hold up well to the blackening seasoning and won't fall apart when cooked.
Mix together your blackening seasoning. This typically includes a combination of spices like paprika, cayenne pepper, garlic powder, onion powder, thyme, oregano, salt, and black pepper. Adjust the amounts according to your taste preferences and spice tolerance.
Pat the fish fillets dry with paper towels to remove any excess moisture. Then, generously coat both sides of the fish with the blackening seasoning mixture. Make sure to press the seasoning into the fish to ensure it adheres well.
Preheat a cast-iron skillet or a heavy-bottomed frying pan over medium-high heat. You want the pan to be hot enough to sear the fish and create a flavorful crust.
Once the pan is hot, add a small amount of oil (such as vegetable or canola oil) to prevent sticking. Carefully place the seasoned fish fillets in the pan and cook for about 3-4 minutes on each side, or until the fish is cooked through and blackened on the outside.
While the fish is cooking, prepare your taco toppings. Traditional toppings include shredded cabbage or lettuce, diced tomatoes, sliced avocado, chopped cilantro, lime wedges, and a creamy sauce such as chipotle mayo or cilantro lime crema.
Heat the tortillas by placing them directly over a gas flame for a few seconds on each side, or by wrapping them in a damp paper towel and microwaving them for about 30 seconds. This will make them pliable and enhance their flavor.
Once the fish is cooked and the tortillas are warmed, assemble your tacos. Place a blackened fish fillet on each tortilla and top with your desired toppings. Squeeze fresh lime juice over the tacos for an extra burst of flavor, and serve immediately.