Normally blackened fish is made with lots of butter, some butter is smeared on the fish to get the pepper mixture to stick and then butter is thrown into the very hot cast iron frying pan to blacken the fish.
If you're trying to avoid fat, blackened fish or blackened anything is a disaster.
I did try it with no butter at all, and it came out dry and stiff. I used the natural wetness to try to get the pepper mixture to stick and it did not. I used only a bit of butter-garlic flavored (but butter-free) spray to grease a very hot cast iron frying pan. This is clearly no good.
Based on a suggestion in a fat-free cookbook of using non-fat yogurt in place of butter in recipes, I got the idea for a good non-fat way to make blackened fish and tried it last night on some trout filet. It came out fantastic, actually better than the regular kind, in my opinion.
I take thin filets of fish, and spread non-fat yogurt on both sides of each piece of fish. Then I "bread" the filets in the cajun pepper mixture (you might wish to use less pepper mixture, just sprinkling it to your desired amount). In the mean time, I have had a very heavy non-stick frying pan sitting on high heat for 10 minutes (Normally you should not use non-stick, but with the non-fat, it turns out better this way. The frying pan must be very thick so it does not buckle under the heat). When the pan is ready, I spray a bit of the butter-garlic flavored spray, and then throw the filets on. I cover the pan and let the filets cook for 2 minutes on one side. I uncover the pan, flip the filets, cover again and let cook 2 more minutes on the other side. It's important that the filets be thin enough that they cook in 4 minutes. After the second 2 minutes, I remove the pan from the heat, and dish out the filets to the hungry folk gathered at the table.
They come out well blackened and moist. It seems that the yogurt behaves like the missing butter in burning and in sealing in moisture (It's the burnt butter that makes blackened food black, not the peppers.).
Leave the pan sit uncovered until it is cool enough to touch. Don't run it under cold water to cool it down; otherwise you will buckle the pan.