K9 Krazee is correct. Genetically there is no such thing as a pure bred black colored boxer. The dogs that look black are reverse brindles, where the brindling is so faint or small that it's sometimes not noticeable and sometimes is only present in puppies, but they are still reverse brindles genetically. These dogs are also sometimes referred to as Seal Brindles.
http://www.boxerworld.com/coat_colour/
That link will better help you make sense of it.