Coat both sides of fish with cooking spray; sprinkle evenly with 1 1/4 teaspoons blackening seasoning. Place fish on rack of a broiler pan coated with cooking spray. Broil 5 1/2 inches from heat 6 minutes on each side or until fish flakes easily when tested with a fork.
While fish broils, combine mayonnaise, remaining 1/4 teaspoon seasoning, relish, and lemon juice, stirring well. Serve fish immediately with mayonnaise mixture and, if desired, lemon wedges.