I love blue heelers They are very smart and loyal to their owners.
Regarding Carter's nipping, I think it is totally because he was bred to herd cattles so you will need to work hard to correct this behavior towards people.
I believe smart dogs can control themselves even their instinctive behaviors with consistant help from patient owners.
I would carry small pieces of treats all the time and when he nips my heel, I would stop and shout "Ouch!" and look at him. As he gets startled, I would tell him to "Sit" and "Stay" and I would walk away for a few feet. Release him and give him a piece of treat and a lot of praise when he comes back to you. Now start walking again and see if he still looks at your heel, or looks at your face. I would expect his focus would eventually change from "heels" to "faces" by repeating this. I hope this will make sense to you.
I think MHN's advice sounds great! Startling your dog when they exhibit bad behaviours is a great way to regain their attention so you can divert it to obeying commands. Then, when you have their attention again, you can go back to what you were doing.
Just be sure to stick to a repetitious regime of it - do it everytime he nips, not just sometimes. Dogs learn when you do the same reaction to the same stimulus EVERY time.
