She's going to have a beautiful adult coat. I have that in my dogs too. They don't blow their puppy jackets until they are 18 months old and the first blow is whopper ! They are ugly then . It has nothing, I repeat nothing to do with coat color.

I've heard the same, she'll have a beautiful adult coat. I've got a young boy who is By and his undercoat is brown. You can see it slightly as his coat is a bit "browner" them my other blacks coats. Hopefully he'll have a nice adult coat :)

I had one as an adult that kept the brownish under coat. He did not carry chocolate, but was a By. I later found out that his sire was known for producing brindle. Not sure if that had anything to do with his unusual undercoat color or not.

We had a BB, kk (Brindle/black-tan free) and she was bred to a chocolate and "one" daughter has it though not as pronounced.
I have also seen it in a friends Black dogs that would not be related for many years to the 60's. They were BY.
It's just a normal different shade of black.