I am not sure what you mean by "base coat" (the soft underlying hair, perhaps?),but here is my 2-cents-worth. I have never heard of this. I think it is unlikely that such a pup will grow hair that is black through and through. every pup I have ever seen is black from the start. I now have a Landseer (white and black). With Landseers, some small areas that start out white do become black. (You can tell somewhat by the color of the underlying skin whether this will happen or not. The skin under black hair is somewhat bluish and the skin under the white areas is piink.) HOwever, mostly, he looks as he looked at 9 weeks, except for black hair overgrowing his forehead blaze. I would check with your breeder about this. If you did not get your dog from a breeder, I would doubt that it is a purebred Newfoundland, or else you have an interesting mutation.
If you mean undercoat as the base coat then yes to a point. My female has a grey undercoat that pretty much brushes out inth summer. Her coat is very very fluffy. There is different types of Newfoudnlands and this is what people for some reason have a hard time accepting. A true Newf like my male has coarser hair and after swimming he shakes and he is dry. This is the breed that we refer to as a true water Newf. These are the dogs that were and still are used on the boats. I live in Newfoundland and raise Newfs so i am knowledgeable of there backgrounds and there differences. Newfs were mixed with St. Bernards long long ago to try to repopulate the St. Bernards by the monks. Any pup that looked at all like a Newf was discarded. From these discards we now have multicolored Newfs (brown, grey and landseers), My female is one of these. They look prettier no doubt as they are fluffier but they tend to hold water more on there coat. I can send you pics if you would like to explain this better.