If someome tries to tell you different the don't know better.

Here you will find FACTS on the myths of Black Boxers

Here is a link to how the black gene works and how the brindle gene works
This website is part of series on Dog Coat Genetics and was last updated on September 16, 2009 by Sheila Schmutz, Ph.D.

The author writes....those who claim to be breeding solid black boxers often use Frau Stokmann's writings as "proof" that there is such a thing. As is evident, however, Stockkmann makes it quite clear that the black dogs were not purebred Boxers, that the colouring came from another breed, and that they were quickly eliminated from the Boxer gene pool.

Boxer Colours and Markings
Worldwide Boxers-Judy Horton 

Coat Colour Inheritance -Newcastle Boxers