Melt the butter in a medium-size saucepan. Stir in the lemon juice, thyme, basil, black pepper, red pepper, and the salt. Cook over low heat for 10 minutes.
Remove from heat and pour into a shallow dish.
Dip both sides of each fillet into the butter mixture to coat thoroughly. Place the fillets on a plate, cover with plastic wrap, and refrigerate until cold, at least 1 hour. (The success of this technique depends on having well-chilled fish and a very hot skillet. Be prepared for smoke.)
Reserve any remaining butter mixture.
Heat a large cast-iron skillet over high heat until a drop of water sizzles in the pan. Place 2 fish fillets in the skillet and cook quickly on each side to blacken, 1 to 2 minutes per side.
Repeat with remaining fillets.
When all the fillets are cooked, add any remaining butter mixture to the pan and scrape up the browned bits stuck to the pan. Spoon over the fish fillets and garnish with parsley.
Serve immediately with lemon wedges.