To be chocolate they must be homozygous for chocolate (2 alleles for males & 1 allel for females). Chocolates from non chocolate parents would be female.
In your case all male ducklings are carrying a chocolate allele they received from their mother.
And because they would be sex linked, resulting males would be black or blue and females would be chocolate? Just trying to make sure I have this right. I can be thick headed sometimes.