In a small bowl, combine the olive oil, mayonnaise, scallions and relish to make a tartar sauce. Refrigerate until needed.
Heat oil in deep-fryer to 350 degrees F.
Combine cornmeal, Blackfish Spice and salt and pepper, to taste. Dredge catfish fillets in seasoned cornmeal. Deep-fry coated pieces in batches until they are cooked through, about 10 minutes. (For basic reference, use the manufacturer's instructions for your deep fryer for similar foods, then test the fish see that it is fork tender.) Allow to drain on paper towels and serve with tartar sauce and fresh lemon wedges (to squeeze lemon juice over the fish).