It is a genetic impossibility for a pure bred Boxer to throw solid black without a brindle pattern. This brindle pattern should be clear and present, it you have doubts look at the coat in light. The gene for solid black coat color simply does not exist within the boxer gene pool. This is very well documented by canine geneticists. A black boxer is like a brindle Rottweiler: the only way to get one is to breed a boxer with some other breed that does carry that gene (Great Dane, for example). These "black" dogs may be AKC registered, but somewhere in their background (many generations back) an unscrupulous breeder (or perhaps a breeder unaware their dog was bred by a different stud) falsified (intentionally or unintentionally) a litter's registration. And everyone who owns a solid black or seal boxer or a puppy of any breeding from a black/seal parent has become a victim of that fraud/accident.
in 2011, the American Boxer Club published a position statement on the so-called "black Boxers", which includes the following:
"In recent years there have been a growing number of people advertising “black Boxers,” or worse "rare black Boxers," usually at inflated prices. The American Boxer Club condemns this practice because the Boxer breed does not carry a gene for a black coat."
"Some brindle Boxers may be so heavily striped as to appear to have a black ground color, especially as puppies. These dogs may be termed "reverse,” “seal,” or “black brindles,” but responsible breeders will never try to pass them off as black. Nor will they use the term “sealed,” implying that the fawn color is sealed out by the black. The breed standard states that the fawn background must clearly, if barely, show through the black striping. As a dominant color, a black coat cannot lie hidden for generations. Therefore, any Boxer with a solid black coat must have another breed in the background."