Well, quite simply the gene that is responsible for black coat colour does not exist within the purebred boxer gene pool. Since I believe it is a dominant gene, it would be quite obvious if it did. A black boxer is rather like a harlequin rottweiler - the gene responsible for that colour simply is not within the rottweiler gene pool (unless it got there by crossing with another breed).
The (extremely) brief flirtation boxer breeders in Germany had introducing black colour into boxers around 1900 is well documented, as is the subsequent erradication of that gene from the breed. A simple task with a dominant gene

You could take a look at Friederun Stockmann's book "My life with boxers" for more detailed information about that.
More modern good sources of information are Malcolm B Willis' book "Genetics of the Dog" - although it's a while since I read that one, I believe it goes into some detail on the coat colour genetics of the boxer. Clarence C Little is another geneticist who has done some studies of coat colour inheritance in boxers - one of his books is "The Inheritance of Coat Color in Dogs". Those would probably be worth your while looking at. That's what comes to mind immediately anyway.
I have seen one or two website, invariably those of backyard breeders, claiming to have purebred black boxers. They don't. They have mixed breed dogs. I've also seen a few claiming to have "blue" boxers too. Haven't yet seen a harlequin one - but that doesn't mean there isn't some BYB out there claiming to have one and I just haven't yet seen it
On that note though, I'm going to close this thread. The rules here about black boxer threads are quite clear. And the rules are there because such threads invariably become heated, and some unfortunate individuals end up believing such colours exist. There are plenty of old threads in the archives if anyone wishes to revisit the issue. There is also a small amount of information on the genetics of coat colour inheritance (obviously, not including colours not present in the boxer gene pool) at
www.boxerworld.com/coat_colour