In a small bowl, mix together the kosher salt, chili powder, and garlic powder.
Dry the fish with paper towels. Rub the spice mixture all over the fish.
Heat a large (12-inch) nonstick skillet* over medium-high heat, about 3 minutes. Brush with the oil.
Place the salmon fillets in the skillet. Cook until the outside is blackened, 2-3 minutes per side.**
Remove the skillet from the heat and cover it tightly with foil (or use a lid if has a tight-fitting lid). Allow the fish to finish cooking in the pan's residual heat, about 5 minutes. Serve immediately.
*Blackened foods are usually prepared in a cast-iron skillet. However, I have found that fish sometimes sticks to cast iron, so I prefer to use a nonstick skillet. **Assuming you'll serve the fish skin side down, it's best to start cooking the fillets skin side up. The side that's cooked last will have a slightly wet appearance while the side that's cooked first will be more aesthetically pleasing. So you should start with the side that will be visible on the plate. If you plan on serving the fish skin side up, then you should cook the skin side first, as I do in the video. The advantage of this method: it creates crispier skin.