How to Make It
Stir together first 8 ingredients (through cayenne pepper) in a small bowl until well combined.
Sprinkle fish with salt, and let stand 20 minutes. Drizzle with oil.
Heat a large cast-iron or heavy saucepan over high. (And by “heat the pan,” I mean get it screaming hot. Turn on your vent to high, and open some windows. You also can heat the pan on a grill outside.)
Sprinkle fillets evenly with 1/4 cup blackening spice mixture, pressing to adhere. Add butter to skillet, and swirl to coat. Place fillets in hot skillet; cook 2 minutes. Flip fillets and cook 2 to 3 minutes. (The spices will have formed a highly colored crust in varying shades of sunset and midnight black. The fish is done when it flakes apart under the gentle pressure of your finger.)
Remove fillets to serving plates, and sprinkle evenly with fresh herbs. Serve with lemon wedges.