We used to have a horrible time with horseflies. They were so bad that most of our horses had bloody necks and shoulders. We tried spraying them - which worked for the regular flies, but seemed to just make the horseflies all the more determined...and bloodthirsty.
Vet told us to use the yellow(sulfur) salt blocks. After about two weeks, the horse flies completely disappeared. Guess they moved on to greener, less smelly pastures? Actually, though - I didn't notice any difference in the horse's smell after they started on the salt blocks. Guess it was enough for the horseflies. Oddly enough - it also helped with the ticks we've had such a problem with. Didn't get rid of them altogether, but we found about a 75% improvement.
Now we set out the yellow salt blocks in March so the horses have a chance to build up whatever it is that gets into their system that keeps the horseflies away.
There were only two of our horses that weren't bothered by the horseflies - Dancer and Rain. Dancer is a grulla, and Rain is a blood bay, so I don't think it was their color. Dancer is Rains dam, and we also have her sire (who is now a gelding), and he was one of the worst affected.
Plain Old Dee, horses Dancer and Rain I believe in dragons, unicorns, good men and other mythical creatures!