This is very interesting! I think I might know what's going on here.... Here's my guess:
Champagne D'argents have the genotype aa B_ C_ D_ E_ sisi
So for the "D" gene champagnes are "dense" (aka black).
They can either have "DD" with two dominant genes, or "Dd", with a recessive. Since the "D" dense gene is dominant, even if they have a recessive "d" gene they will still be "dense" aka black, and look the same as the others.
Your doe and buck have the genotype Dd. They are black (dense) but hold a recessive "d" dilute gene, which was passed onto this lighter colored offspring, which has the genotype "dd" dilute. This makes their coat color lighter and more of a blue. This would be not desirable for show rabbits, but I bet it will be a beautiful bunny!
This cross would have a 25% chance of producing a dilute "dd" kit.
I would love to see pictures of this kit as it grows and begins to silver. It's super interesting!
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