My dog has a blue eye and a brown eye with a little blue at top,I blame it on genetics.I claim he has a day vision eye and a night vision eye and he's afraid of the dark.LOL My Roman Tufted geese have blue eyes so maybe it has blue-eye genes from another breed used in previous breedings and it showed up in that duck.
It is cool! Yes, it's genetic material that shows up every once in a while. It used to be considered an undesirable trait for judging, but these days it's an interesting anomaly.