Australian Cattle Dog/Blue Heeler Question

Blue eye color is a trait common when domesticating dogs, some breeds have it some do not. It is very common in Huskies and in Australian Shepherds. It is very common to mix ACDs with Aust Shephereds, since the breed standard for ACD says only brown eyes, and ALL the purebreds ive seen have only brown eyes, it is probably a mix that happened way earlier in the bloodline and is showing up now, either way it means AT SOME point the bloodline was mixed.

I also have a heeler with one brown and one blue eye. But my dog is a mix between a bearded collie and a red heeler. Both of his parents are pure bred but because he is mixed he got that trait.

Our pups parents were both pure bred blue heelers. Must be a recessive gene from lineage some where...