Black is genetically impossible in boxers. They are either heavy brindle (reverse brindle..and some people called sealed brindle, which is not a correct term either) or they are mutts.
So called black boxer breeders have started breeding the Heavily brindled boxers to other Heavily brindled boxers getting solid brindles....which look black to the average eye, or even that they are so heavily brindled that you can't tell unless in the sunlight....
But no, black boxers are NOT possible.
(and those breeders that use the frau stockman book and such haven't read or comprehended it either....)