1. For salsa, mix all ingredients together and set aside. For Cilatro Lime Mayo: In a small bowl, combine all ingredients. Cover and chill.
2. In a shallow dish, combine first 6 ingredients.
3. Pour melted butter into another shallow dish. Dip fish fillets in melted butter and coat with seasoning mixture.
4. Heat a cast iron skillet over medium-high heat until hot.
5. Cook prepared fish, in batches if necessary, 3 to 4 minutes per side, or until fish flakes easily with a fork.
7. Spread Cilantro Lime Mayonnaise evenly over toasted buns.
8. Place blackened fish on bottom half of bun, top with mango salsa, and cover with top half of bun.