Horses frequently kill injure or kill dogs or coyotes. We used to pull up lawn chairs to watch the trainer's horse go like murder after the trainer's dog (we were forbidden to do anything about it). That dog had horse shoe marks on its back nine days out of ten. Eventually it was badly injured and required several surgeries. We never could fathom the logic of the situation.
Having a blue heeler work the horse is not advisable. The dog will probably wind up dead or badly hurt. Heelers get kicked by horses, so do most other dogs, and very frequently. Horses and canines do not mix well. Smaller or younger horses may be panicked and run into fences or trees with severe injuries, larger horses usually turn and fight.
You might want to tell your barn manager so s/he can clean up the area - animal carcasses tend to be associated with botulism toxin in warm parts of the country during rainy seasons.