Many wonder if perhaps there were black Boxers that were mostly bred out of the lines and that perhaps now and again a dog with a black coat shows up somewhere. However, if we look to the record keeping of the past century, we can see that this is not the case. In this 100 year time period, a black Boxer appeared once, however there is a catch to this.
In the Germany, in the late 1800's a Boxer was paired with a mixed dog that was part Bulldog and part Schnauzer. The resulting litter had puppies that had black coats. Since another breed was introduced into the bloodline, they were not purebreds. These dogs were not used for any further breeding and therefore did not have any influence on genetics going forward.
Once in a while there will be a breeder that claims to have black Boxers and will point to this one incident long ago as evidence that black does indeed run in the bloodline. However, since those mixed dogs with black coats were never used for any sort of develop program this simply does not hold true.
Another element that shows that this coloring does not exist in the Boxer line is the rule that the Munich Boxer Club created in 1925. This group had stringent control over breeding and development of Boxers in Germany and set the guidelines for the standard, conformation and all elements regarding appearance including coloring.