In a shallow dish, combine spices.
Pour melted butter into another shallow dish. Dip fish fillets in melted butter and coat with seasoning mixture.
Heat a cast iron skillet over medium-high heat until hot. Cook prepared fish, in batches if necessary, 3 to 4 minutes per side, or until fish flakes easily with a fork.
Spread Cilantro Lime Mayonnaise evenly over toasted buns. Place blackened fish on bottom half of bun, top with mango salsa, and cover with top half of bun.
Cilantro Lime Mayonnaise:
In a small bowl, combine mayonnaise, cilantro and lime juice. Cover and chill.