Ktibb, I was more concerned with the long lead rope trailing on the ground than the leaf blower, lol!
I think desensitising your horse to such things is a great idea. The only thing I would have done differently is I would have rubbed the leaf blower all over his body before I turned it on, and then again after!
We were having a silly games day at a friends barn once, and wanted to blay broom ball on horseback. Spike was freaking, so I asked for a 5 minute break. First I stood by him with the broom like I was sweepin, then started sweeping him, his back, his face, and was spinning the broom high up right in front of him. Then I took the ball, rubbed it on him, and was eventually bouncing it off his nose from 6 feet away (it was just one of those super light balls like you find in those huge bins at walmart). Resumed game, and we won, lol! I think they call it sacking out. I've taken him right up to log splitters, chainsaws, and campfires at every opportunity. You never know what you might meet out on trail!