Melt butter in medium saucepan over low heat. Stir in lemon juice through kosher salt. Cook over low heat for 10 minutes; pour into shallow dish.
Dip fillets into butter mixture, coating thoroughly. Place fillets on platter; cover and chill at least 1 hour. Set aside remaining butter mixture.
Heat a large cast-iron skillet over high heat until a drop of water sizzles in the pan. Add 2 fillets, and cook 2 minutes on each side or until fish flakes with a fork. Keep cooked fillets warm. Repeat procedure with remaining fillets.
Reduce heat; add remaining butter mixture to skillet, and stir well. Drizzle butter mixture over each serving. Sprinkle with parsley and serve with lemon wedges.
Note: The success of the blackening technique depends on having well-chilled fish and a very hot skillet. Prepard only in a well-ventilated area.