Make the blackening seasoning: In a small bowl, combine the salt, smoked paprika, thyme, oregano, garlic powder, onion powder, mustard powder and cayenne.
Carefully slice the chicken breasts horizontally in half, then pat dry with a paper towel. Thoroughly coat all sides with the blackening seasoning and set aside.
Drizzle oil into a large skillet or cast iron pan over medium-high. When the oil begins to shimmer, carefully place the chicken in the pan and cook undisturbed for 5 minutes. Watch out for any oil splattering and popping out of the pan. Cook until the seasoning literally darkens. After 5 minutes, flip the chicken and cook for another 3 minutes.
Turn off the heat and leave the pan tented with aluminum foil for 5 minutes (this will help the chicken continue cooking from the residual heat without drying out). Remove the foil, then let the chicken rest for at least 2 minutes.
When ready to serve, slice the chicken on a bias into ½-inch pieces. Serve immediately.
