Blackening is exactly as Scovie posted.
Dredge the fish in melted butter, then press the fish into the spice rub (or pat the spices on like Scovie) for an even coating. Then, sear in a CI pan on medium-high heat, flipping when the edges of the fish start to rise from the pan, around 1-2 minutes per side depending on size.
NO oil ever. The point of the butter dredge is that it also acts as the cooking medium, but it is under the spices, so the first contact with the hot pan is the spices, which form a crust as the butter quickly soaks into them for a perfect golden brown crust with some char.
Scovie's method above is SPOT ON but I don't like my pan that hot, I prefer a medium-high heat as to not singe the spices upon contact. On medium-high it gives the butter a chance to soak in so less char, but his method is 100% correct. THAT is blackening and don't let no one tell you different!