In a small bowl, whisk together the paprika, thyme, onion powder, garlic powder, cayenne, and kosher salt.
Brush (or spray) the chicken breasts with the oil on both sides, then sprinkle both sides with the seasoning mixture, pressing with your fingers to help the coating adhere.
Heat a well-seasoned, large cast iron skillet over high heat until smoking hot, about 5 minutes.
Add the chicken breasts. Cook 30 seconds on each side to blacken, then place the skillet in the oven to finish cooking, 10-12 minutes. Chicken is done when no longer pink in the center, when juices run clear when pierced with a fork, and when the internal temperature reaches 160 degrees F.*
Remove the blackened chicken breasts to a platter, loosely cover them with foil to keep them warm, and allow them to rest for 5 minutes before slicing and serving.
*If your chicken breasts are larger than 8 oz each, they will need more time in the oven - around 15 minutes. In this case, loosely cover the pan with foil to prevent their tops from burning.