In a small bowl, whisk together the smoked paprika, garlic powder, onion powder, dried thyme, dried oregano, cayenne pepper, sea salt, and black pepper to create the blackened seasoning.
Heat 1 teaspoon of olive oil in a medium skillet over medium heat and sauté the finely diced onion, green bell pepper, and celery until the vegetables are tender and translucent.
Add the cooked brown rice to the skillet with the vegetables, sprinkle with a pinch of the spice blend, and stir until well combined and heated through.
Pat the catfish fillets completely dry with a paper towel and coat both sides generously with the remaining blackened spice rub, pressing it into the flesh.
Heat the remaining 1 teaspoon of olive oil in a separate cast-iron skillet over medium-high heat until the pan is very hot and the oil is shimmering.
Place the catfish fillets in the hot skillet and sear for 3 to 4 minutes per side until a dark crust forms and the fish is opaque and flakes easily.
Divide the warm Cajun rice between plates and top with the blackened catfish fillets, serving immediately while the crust is crisp.