Agreed, dry is the only way to go. I left it on the grill for 7-10 minutes with the cover down (except for the skillet handle protrusion), and it barely produced a wisp of smoke. Perhaps leaving it on longer would have produced the desired results. The salmon and chicken were the last things I cooked, after hamburgers, pork chops, hot dogs, corn and other vegetables, so I was ready to get through and eat. I should handle the blackening first next time, to give the skillet more time to heat up. Their grill is 6 years old, and still using the original burners, so that could be an issue as well.
I'm in the market for a gas BBQ grill and would like to properly blacken chicken and salmon on it from time to time. I tried this Memorial Day weekend on my parents' grill using a 12 inch cast iron skillet. The skillet couldn't get hot enough to blacken the chicken, and merely broiled it, like if I had just tossed it on the grill grate. If you've ever blackened chicken and seen the white smoke pour up from the skillet, you know how much better that this tastes than mere broiling. I do not know what BTU grill they have.
Anyway, how many BTUs should I look for in a gas BBQ grill to ensure I can easily blacken poultry and fish in a cast iron skillet?