How could any of the children of Noah and Allie have kids with black hair?

Your question is vague about whether you are asking about the children or the grandchildren of Noah and Allie. I assume you are asking about the grandchildren.

Did Noah and Allie's children all commit incest, so that Noah and Allie are the only grandparents of their grandchildren?

If not, the spouses of Noah and Allie's children introduced new genes, including new hair color genes, into the family.

Black hair genes are usually dominant, while blonde hair genes are recessive. If a person has a black hair gene from one parent and a blonde hair gene from another parent, they won't have blonde hair, since it takes blonde genes from both parents to have blonde hair. Instead they will have black hair genes, since it takes only one black hair gene from one parent to have black hair.

Thus I suppose that the black haired grandchildren all have parents from outside the family, in-laws of Noah and Allie, with black hair. If any of the sons-in-law or daughters-in-law are ever shown with black hair, it would be impossible for them to have children with blonde hair. In-laws with black hair would have to have children with black hair.

An out of universe explanation may be that the casting director didn't care much about family resemblance when casting the grandchildren.

One the other hand, if you are asking about the children of Noah and Allie, then the only way it would be possible for the children to have black hair is if they were adopted, or if Noah and/or Allie had black hair and dyed their hair blonde.