Leaving the skin on the salmon, trim the fillets to even sizes and thicknesses. This will help for even grilling. Pat any additional moisture away with a paper towel.
Season the flesh evenly with blackening seasoning, garlic, and salt.
Preheat a grill or griddle to 350 degrees F (175 degrees C). Place the salmon on the grill skin side down; let cook for 2 to 3 minutes.
Lightly spray the top of the fish with oil. Turn the salmon with a spatula. Sprinkle again with the blackening rub, garlic, and salt.
Finally, flip one more time to the original side. Grill just long enough to put a small char on the remaining flesh, 1 to 2 minutes.
Serve the blackened salmon with an avocado-based sauce.