heat up a cast iron skillet on the grill (this will get smoky) the pan needs to be hot enought that when you toss a little butter in, it catches fire.
While the pan is getting hot. Coat the fish in your blackening spices, just fish no oil (I have not used the Redfish Magic) I use Tony Cachere's Spice N Herbs mix.
http://www.tonychachere.com/seasoning-blends.asp
Then when the pan is hot, put the seasoned fish in, and drop a small amount of butter in the pan away from the fish.
cook about 45 seconds and flip. Cook another 45 seconds. Remove from heat. (Use a spatula, tongs will break the spices and may tear the fish.)
Open beer, dig in.
When you cook the fish nearly dry, with out oil, the seasonings make a crust on the fish.