Pat the salmon fillets dry and sprinkle both sides with 1 Tbsp Cajun seasoning and the sea salt.
Heat 1.5 Tbsp avocado oil in a 10-inch cast iron skillet over medium heat until very hot but not smoking.
Place salmon in the skillet flesh-side down and cook undisturbed for 3 minutes.
Carefully flip the fillets and cook on the skin side for 6 minutes, then continue cooking and flipping as needed until salmon is cooked through (an additional 2–10 minutes depending on thickness).
Remove the skillet from heat and transfer the salmon to a plate; let the skillet cool until it is no longer smoking.
Return the skillet to the burner over low heat, add 1 Tbsp avocado oil and the minced garlic, and sauté, stirring frequently, until the garlic softens, about 2 minutes.
Add the chopped kale, 1 tsp lemon zest, and 1/2 tsp Cajun seasoning; cover and cook until the kale wilts, about 2 minutes.
Remove the cover and continue cooking over low heat until excess liquid evaporates and the kale is slightly browned.
Serve the blackened salmon with the garlicky Cajun kale.