Mix all dry ingredients well and rub on tuna steaks. Heat a cast iron pan until very hot. Melt butter and add the tuna. For 1/2″-thick steaks, blacken 2 minutes per side. For 3/4″ steaks, blacken 3 minutes per side.
You can cut a piece to check for doneness. It is better to undercook than to overcook, as the fish will continue to steam when removed from the heat.