Prepare an indoor or outdoor grill.
Mix together paprika, chili powder, oregano, thyme, cayenne pepper (optional), ground black pepper, garlic powder, and salt (optional)
Rub each tuna filet with blackening seasoning well on both sides
Spray the tuna filets with cooking spray and grill 4-5 minutes on each side. Tuna should be slightly pink in the middle.
In a small bowl, whisk together yogurt, mustard and soy sauce. Divide sauce evenly among the 4 tuna filets and top right before serving and sprinkle with chopped fresh parsley.