There is no better fish in the world for blackening. I put a cast iron skillet directly above the burner on my grill, take the grate off and set the pan on the stones. Turn the gas up as high as it will go and go find something to do for 20 minutes. You want the pan to be screaching hot. Lightly coat the swordfish in olive oil and sprinkle it with garlic powder, paprika, cayanne and salt and pepper. Lay the fish on the pan and stand nearby. It will produce a LOT of smoke. (That is why I only do this outdoors on the grill.)
I always buy a really thick piece of swordfish if I'm going to be cooking it this way. IMHO, anyone who overcooks swordfish is a criminal and should punished harshly and it's really easy to overcook it doing it this way. Aim for medium rare and you will probably end up with meduim. If I'm cooking a 1" thick slab I'll probably go for 1 1/2 minutes on the first side and no more than 1 minute on the second. Again, that cast iron skillet wants to be burning down teh house hot before you put the fish on.
Leave the pan on the rocks for a while afterwards, it takes a long time to cool down to the point where you can take it out without risking burning yourself thru the oven mitt.
And if you can get it, ALWAYS buy harpooned swordfish over hook and line swordfish. It's a night and day difference. You need to really trust your fishmonger to know what he is passing on to pay for the difference.
PTN