Combine all the ingredients for the salsa in a medium bowl. Cover tightly with with plastic wrap and refrigerate until ready to use.
In a small bowl, combine the chili powder, smoked paprika, black pepper, cumin, garlic powder, and salt. Pat the fish fillets dry with paper towels. Brush the fillets all over with 2 teaspoons of the oil and season with the spice mixture.
Heat a large ovenproof skillet (preferably cast-iron) over medium-high heat. When the pan is piping hot, add the remaining 1 tablespoon oil and swirl to coat the bottom of the pan. Add the fillets to the pan, skinned side up, and cook for 1 minute, or until lightly "blackened" (read: deep reddish-brown) on their undersides. The spices and oil will smoke. That's part of the "blackening" process - crack open a window or turn on the vent fan and don't panic.
Carefully flip the fillets and cook for 1 minute more, or until equally blackened on the opposite side. Pour the lime juice over the fish and transfer skillet to the oven. Bake for 6-8 minutes, until the fish is just opaque in the center and flakes easily with a fork.
Serve your blackened fish warm, topped with plenty of salsa. Enjoy!