Genetics in a nutshell.
Eye pigment is made up out of 0-6 dominant genes. The parents can pass on the sum of both of their dominant genes at maximum:
0 - light blue; 1 - grey-blue; 2 grey/blue-green; 3 - green to hazel; 4 - medium brown; 5 - dark brown; 6 - black.
Hance two parents with blue-green eyes could have children that have an eye color between light blue and medium brown.
Hair pigment is mainly made up of Melanine. There you have Eumelanine and Phomelanine, but it is enough to concentrate Eumelanine since it answers the question.
Assume for the sake fo it, that half of the genes determining hair colour come from the mother and father and that each of them pass on 4 genes...now let's use "h" for light pigment and "H" for dark pigment.
hhhhhhhh - white blonde
HHHHHHHH - black
Now assuming that both parents are dark blonde (HHHhhhhh - HHHhhhhh) - [which could well be possible, if their parents were both middle-blonde then they have a good chance to have anything up to light brown[ - all combinations between white blonde (hhhhhhhh) and darkish medium brown (HHHHHHhh) become possible.
In a way this could work the other way round as well. I will use a family example to highlight this, in fact that is what happened with the sister of my father: Grandmother, dark brown (HHHHHHhh) + Grandfather, light brown (HHHHhhhh) = Aunt, medium blonde (HHhhhhhh)
Hence, it becomes possible that the children of two blonde, blue-eyed parents can indeed be brown-eyed and brown-haired. Now even assuming that all of their ancestors were of pure Nordid phenotype (whose typical pigmentation is assumed to be medium blonde), so will the children be of pure Nordid phenotype (there are no other racial influences for them) - since, even though the children will have "wrong" pigment, the facial and bodily features will be manifestly Nordid.
Anything else is a simplification of genetics to push an agenda.
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