The Boxer dog does not carry the gene for a black coat color. There are two coat colors for the Boxer dog, fawn and brindle. “Brindle” refers to a dog with black stripes on a fawn background. Brindle is actually a striping pattern, not a color – all brindle dogs have a fawn ground color. However, there are some very dark reverse brindle Boxers that appear to be black but what they have are dark brindle coats that have so many dark brindle stripes that it appears to make the coat completely black. It is not really a black dog; the dog just has an excessive number of black stripes. Some of these brindles may end up with such a great amount of black striping that it can be quite difficult to find the fawn under-color. These Boxers are sometimes said to have “reverse brindling”, fawn stripes on a black body; these dogs are conventionally called “reverse brindles”, but that is actually a misnomer—they are still fawn dogs with black stripes. In addition, the breed standards state that the fawn background must clearly contrast with or show through the brindling, so a dog that is too heavily brindled may be disqualified by the breed standard. These heavily brindled Boxers may also be called sealed black brindles, sealed brindles, double reverse brindles or black Boxers.
Genes control coat color in dogs. In order for a dog to be black, that breed of dog must have the gene for a black coat. Boxer dogs do not have that gene. So, there cannot be any black Boxer dogs. It is genetically impossible. Boxer experts know that there is fawn underneath; even if you cannot see it without close inspection. If a breeder insists that they breed “rare” or “pure” black Boxers they are either inexperienced as to the genetics related to Boxer coat color and markings or they are not breeding purebred Boxers.
This is by no means a complete lesson in Boxer coat genetics but is a summary of what was compiled from available Boxer coat/color genetics information. More in depth information can easily be found with an internet search related to Boxer dog coat colors and markings.