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.