I have a blue cockerel that I hatched from my original line birds that is showing some color like that on his chest. The parent stock does not have it. I have seen pictures of other Isbars that have it, but it is not as noticeable on the blue and splash birds due to the lighter color. I suspect it is along the same lines as Black Copper Marans that are over pigmented and develop coloring on the chest. His build is stockier than my cockerels, but that could be the new line.
BTW - "Legitimate" mistakes that happen over and over and over again with the same breeder no longer qualify as legitimate in my book.