Two blue-eyed parents can have children with brown, green, blue or hazel eyes. It just depends on the recessive genes each parent carries from their own parents. My 6th grade science teacher had 4 daughters with blue eyes despite she and her husband having brown eyes. She used that example to teach us basic Mendelian genetics.
My mother's parents were German-born blue eyed, fair ppl - they had 12 children, 9 of whom had blue eyes, one hazel, and two brown.
Imo I don't see the probability of Dylan's creepo narcissist dad being responsible for Delphi. Filicide is a whole different kettle of fish imo. Is there actually any evidence that he visited that area to visit a relative? Bc that seems unlikely. And quite honestly, I think Libby's grandfather looks more like BG than he does, and I don't think thr grandfather did this either.