As for scents, many purists state that game is spooked by man-made smells. This IS true, but more importantly pay attention to the wind. It's direction will change with the time (temperature) of the day.
As for black flies, and this has something to do with the statement above since none of the insect repellants are "natural", Bounce sheets used in the clothes dryer are very effective. Not so much for mosquitoes though.