Abrown eyed person marries another brown eyed person, but they have a blue eyed child.

One of there ansestors must have had blue eyes, or might have been a genetic mutation. But it's rare to find such a random mutation. But we must keep in mind that reproduction of cells and when DNA is copied it could have a mutation. But the likely explanation would be that one of there ansestors or grand parents had blue eyes until the trait that forms brown eyes became more common in the family tree.