1. Combine pepper, cayenne, celery salt, paprika, garlic powder and thyme in a shallow bowl; mix well.
2. Warm a large cast-iron skillet over high heat until hot. Brush fillets with butter and coat on both sides with spice mixture.
3. Working in batches, cook fish, turning once, until blackened on both sides and flaky inside, 3 to 4 minutes total. Serve immediately.