Found this on http://fooddownunder.com/cgi-bin/recipe.cgi?r=41427 . I made it last night and it was pretty good.

Recipe 41427 of 281476

Title: Cajun Blackened Tuna
Keys: Fish Ocean Main Dish Seafood
Yield: 4

Ingredients:


----------------- SPICE MIXTURE ----------------
1 tbl sweet paprika
1 tsp oregano
1 tsp garlic powder
1 tsp onion powder
1 tsp salt
1/2 tsp ground cumin
1/2 tsp freshly-ground black pepper
1/4 tsp cayenne pepper (optional)

----------------- TUNA ----------------
2 tbl unsalted butter softened
4 x tuna steaks, abt 2/3" thick - (8 oz ea)


Method:
Heat oven to 400 degrees. Combine paprika, oregano, garlic powder, onion powder, salt, cumin, pepper, and cayenne pepper on a plate. Rub butter over tuna steaks. Press tuna steaks into spice mixture; gently rub spices onto fish.

Heat a large heavy ovenproof skillet (cast-iron works great) over high heat for 2 minutes or until skillet smokes. Cook tuna steaks 1 minute on each side. They will make some smoke, this is normal. Transfer skillet to oven and roast fish 5 minutes for medium-rare doneness.

This recipe yields 4 servings.

Comments: If you dont have an ovenproof skillet, transfer seared fish to a baking sheet to finish cooking.

Description: "Tuna, a naturally lean fish, is tender and juicy when cooked rare or medium-rare. In this dish, searing dry heat chars a spice crust onto the fish."