They are used for cattle, but not sheep - not sure why, but those who use and breed these dogs are very keen on all the right traits. Kelpies are used for sheep.
You should be aware that for some odd reason, the colour red is called blue in Australia. So men with red hair are often nicknamed 'Blue'. Thus the Blue Heeler is connected to a common reddish colour they often have.
Julie's last dog - Jethro - was a specially intelligent dog, and especially bread for cattle work. He was basically a Collie, for intelligence, with three other breeds mixed in all for various traits, including Blue Heeler. I can recall he had some Bull Terrier. Collies are not so good at Australian Sheep herding as they are bread to go in front as they do in Europe, but sheep dogs generally will scurry around the rear of the sheep. Bull Terriers have a trait of going in head to head, which is what they need for cattle work - the dog has to attack the cattle from the front, as cattle will turn and attack from the head-horns. So Jethro had the ability, which we discovered when we accidentally called out the right words one day.
But poor jethro was never very keen on that sort of thing - he did it, but you could see it was a worry for him. What he liked best was rooting - no dog could touch him for slipping his chain and going off in the night to service all the bitches on heat in the neighbourhood.