In a small bowl, combine the ingredients for the blackened fish seasoning.
Sprinkle tilapia with the blackened seasoning. Sprinkle more if you like things spicy and less if you do not. Using your hands, gently press the seasoning into the fish.
Add oil and butter to a large skillet and set it over medium-high heat. Carefully add the tilapia to the skillet and cook 4-6 minutes on each side, using a spatula to flip the fish. (The tilapia will fall apart if you try to pick it up with tongs.) Sprinkle fish with the lemon juice and transfer to a plate.
Reduce heat on stove-top to low and add blue cheese and blue cheese dressing to the skillet, mixing constantly. Only allow the sauce to cook for about 30 seconds or less, scraping the bits at the bottom of the skillet into the sauce. Cooking the sauce too long will scorch the sauce, so this step should be quick. Pour sauce into a small bowl and stir the sauce with a spoon.
Top each piece of blackened tilapia with approximately 2 teaspoons of the blue cheese sauce.