Philip of Spain's Conquistadores, who came to the Americas , frequently carried " mixed blood" being descendents of Moorish matings and during the occupation of the Netherlands , Jewish heiresses were often forced to convert to Catholicism and were married to Spanish soldiers as a "reward" to their service to Philip. It's not unusual for DNA reports of presumed white Christian people to get a listing of small percentages of African and Sephardic genes, particularly in Mediterranean and Latin America populations. Two blonde, blue-eyed, "white" parents who share such descents could have a child where the "dark" recessive emerges .
Consider the Cajuns and Creoles.