Here's one I found with a Google search.
http://www.recipecottage.com/cajun/blackened-redfish04.htmlI do mine similarly. I use this stuff:
They make several flavors. They have one called 'Blackening Seasoning' and one called 'Creole Seasoning'. I prefer the creole, which is hotter. Sparkly likes the blackening, cuz its not as hot. You don't need to use this. Any cajun seasoning will do .... or you can make your own mix.
Pat the fillet dry. Coat it completely with lots and lots of the seasoning.
Heat a cast iron skillet until it is (almost literally) red hot.
Put the fish in and let it cook, undisturbed. You want it to sear and form a crust. If the pan's well seasoned, it will stick at first and then release. If the pan is not so well seasoned, use a tiny bit frying oil that has a very high smoke point.
Your pan will smoke like a tire fire. Open windows, set up fans, cover your smoke alarm ....... or do it outside over a very hot fire!
Wait about 2 - 4 minutes (depending on the fish's thickness) then turn. I use a stiff metal spatula so I can sorta scrape under the fish to get under any part of the crust that may be stuck.
Cook about 2 - 3 minutes on the other side. Poke into the fish with a fork to check for doneness. When done serve immediately.
The fish will look .... well ... black. This picture's close. Mine gets more evenly black.