I didn't do any researching before replying to you, but from what I know, darker hair is normally a dominant trait. This means that if this gene was inherited at all by either parent, their hair would've been dark.
Blue eyes are a recessive trait and brown are dominant. This means that if you get genes for both blue and brown, only brown would show because it 'overpowers' the blue gene.
But you may be right about skipping generation thing. Still a bit odd that the characters themselves acknowledged that Dojun's hair color is different though. Maybe the manhwa was supposed to go in a different direction. Feels like it fell flat on that note.
Why did they censor the mom's answer about the brother? Could they be not blood- related? Plot wise, I don't see any reason to censor if the answer is yes, they are brothers.
Maybe they're not, but the mom wants to keep it a secret for some reason. It is true that Dojun's hair color is different and can't possible be that dark when neither parent has black hair.
And I know you guys don't like the side story, but this just made me want to read Dohwa's story because it's really heartbreaking to see that he ends up alone. He can't choose who he loves.