I stared training my English Shepherd at 12 weeks on blood trails of squirrels I'd shoot for her (dragged them on a string from a pole and let her find them). She tracked her first deer at 9 months and has since tracked moose, deer, bear, caribou, coyotes and other stuff I can't even remember. I let her find anything I shoot. If I let her track off lead she'll run ahead but comes back and finds me until we get there...kind of like hunting with Lassie.
I think any dog with some level of prey drive and biddability can be trained to blood trail. But too much prey drive might even be a detriment in some ways as the dog might be harder to handle and live with when you're not hunting. My dog is always ready to hunt when I am, but isn't totally crazy about it when we're not hunting and will leave stuff alone when I tell her ("No, we're not squirrel hunting right now").
