Gary, here's a quick scoop:
Blackening is the process of cooking fish by coating in spice and cooking at extremely high heat. The process actually creates a barrier between the food and the heat, allowing blackening, not burning, to occur.
To blacken properly, a cast iron skillet or grill plate is heated over very high temperature until just short of a white spot, or ash appearing (the pan should be dry). The fish to be grilled should be at room temperature. Coat fish with spices just before cooking. Place the fish onto the pan, cooking for 1-2 minutes on each side. Blackening adds a distinctive flavor to fish while creating a savoury, moisture-retaining barrier around the outside. This should produce a warm, smoky grilled flavor and succulent texture. The high heat required by blackening means that you will need a skillet or pan made of cast iron, there is really no substitute. If using a thick peice of fish (such as salmon) you may want to blacken both sides, then finish in the oven to prevent burning.
This is a very enjoyable way to eat fish (squeeze some fresh lemon on top just before eating). Any fish will do, catfish is the "classic" fish for blackening. Hope this is usefull...