Blackened fish was invented by Paul Prudhomme to try to duplicate the flavor and style of charcoal grilling in a commercial kitchen. He describes the method very well in at least two of his cookbooks. The one I have is called something like the Prudhomme family cookbook and is from around 1987. Many chain restaurants have spoiled the name and style of blackening by dredging fish and other meats in spices, frying it in a pan and calling it blackening. Unless you are at a good quality La. style restaurant, you are not likely getting the real deal.
The blackening comes from the butter charring in the pan, not from the spices. When the butter chars the instant it lands in the pan, it creates a sealing crust on the fish that keeps it from drying out.
You need a cast iron skillet, nothing else will do. If you are feeding a crowd, you can use a large 12+ inch one if you have a big enough heat source. The original method is for single servings in a small cast iron skillet as would be done in a commercial kitchen.
If you have been frying fish, you likely have a perfect heat source if you have one of those outdoor frying stands as are sold for Turkey or fish frying. Most gas grills do not generate enough heat. Do not try to do it in your kitchen. My first attempt was indoors back when I was single and I figured it can't be too bad. The book tells you not to do it indoors for good reason, the smoke it generates is prodigious and nasty.
If you only have the gas grill, you have two options. One, take the cooking grate off your grill. Turn it on as high as it will go. Take a large cast iron skillet and heat it up on your stove inside as hot as it can get, until you see a little white ash on the seasoning of the pan. Carry it out carefully and place it directly on the rocks in your grill. Start cooking.
The second option is to take the the little gas jets and bore them out slightly larger so the grill runs hotter. This is probably against some governmental regulations, but it works. Again,
place the pan directly on the rocks.
If you have a gas fryer stand and a big enough cast iron pan to fit on it without burning yourself, that is the best option. Get it hot enough to see the white ash and get started. In this setup, you do have to watch the heat to make sure you don't get it too hot.
The main part of the cooking - have your fish be about 1/2 to 3/4 inch thick if possible. If it is thicker, it will be tough to get it cooked right without burning. Dredge the fish completely in melted butter. Sprinkle a spice mix moderately on the fish. Paul Prudhomme's mixes are good, but you can use whatever you like. The spice is not the key part of the cooking method. Do not put the fish down while sprinkling or at any time between the butter dredging and hitting the pan. Lay the fish in the pan and immediately spoon a tablespoon of melted butter over the top of it. Be careful, it sometimes flames up.
Cook 2-3 minutes and then flip. Spoon another tablespoon of butter over the fish. Cook another 2-3 minutes and it's done.
Like some have said above, I like this the best on firmer fish like amberjack. It is a fairly violent form of cooking and fish like snapper that are more delicate can't take as much abuse as something like amberjack.
My personal favorite recipe for snapper is as follows and is painfully simple. Modified this slightly from the great cookbook "How to Cook Everything." Soak the snapper in cold milk for a few minutes. Mix some plain all purpose flour with your favorite seasoning. Heat a few tablespoons of vegetable oil such as canola oil in a non-stick frying pan on medium high. Take the fish out of the milk and dredge it in the flour. Put it in the pan and cook 10 minutes per inch of fish thickness, flipping halfway through. Example - 1/2 inch thick fish, 2 1/2 minutes on each side.
With the acquisition of my blast furnace grill, I have adapted this a bit and can get results pretty close to the cast iron pan method on the grill. (Note that the original invention by Paul Prudhomme was to try to recreate a grilled fish taste in a commercial kitchen.)
Anyway, here is what I do on the grill.
Prepare and plan just like above, but get the grill smoking hot instead. You need to have a grill that can get REALLY hot. Will probably work for anybody that has a high end grill like those typically found in a BBQ island. It will not work that well on a regular CharBroil from HD.
Melt the butter in a frying pan on a side burner or on a less hot part of the grill. Dip the fish in the butter and season just like above and simply throw on the grill. Assuming you have some melted butter left in the pan…drizzle it on top of each piece of fish after you turn the fish over.
I have been using Zatarain’s or Tony Chachere’s creole seasoning for my spices since they are what's available in my current location in California.