Add the cabbage, cilantro, green onion, jalapeno, mayonnaise, lime juice, sugar, salt, and pepper to a medium bowl and mix with a wooden spoon or rubber spatula until all of the ingredients are combined. Refrigerate the slaw while you prepare the fish.
Heat two tablespoons of olive in a medium cast-iron skillet over medium-high heat. Rinse and pat the redfish fillets dry with paper towels.
Brush each piece of fish with melted butter on one side and generously coat with the blackening seasoning, rubbing it into the fish with your hands. Repeat on the other side of the fish fillets.
Immediately place the fish in the hot skillet and cook for 3-5 minutes until a dark crust forms. Flip the fish and cook for another 2-3 minutes until the fish is cooked through. Transfer the fish to a paper towel-lined plate.
Heat a medium nonstick skillet over medium-high heat and toast the tortillas until warm and browned around the edges.
Lay the tortillas out flat on a serving plate and top with flaked pieces of blackened redfish. Use tongs to top the fish tacos with coleslaw. Garnish with fresh chopped cilantro and enjoy!