Preheat the Blackstone griddle to medium-high (375–400°F) for 8–10 minutes. Pat the catfish fillets completely dry with paper towels.
Season the fillets: for the Cajun version, lightly oil and sprinkle both sides with the seasoning, salt, and pepper. For the blackened version, stir the rub together and dredge each fillet, shaking off excess.
Spread the oil on the griddle, then add the butter and let it melt and foam.
Lay the fillets down spaced apart. Cook 4–5 minutes without moving them, until a crust forms and they release.
Flip once and cook 3–4 minutes more, until the fish flakes easily and reaches 145°F (63°C) internally.
Transfer to a platter, squeeze lemon over the top, garnish with parsley, and serve immediately.