Umm. Sounds extremely
dubious to me.
1. "Reverse" brindle is not a colour that is any different to any other shade of brindle. It is a term used in North America simply to describe a dog that has such a heavy concentration of brindle stripes so as to have the appearance of being a dark dog with fawn stripes (as opposed to a fawn dog with dark stripes). But that is appearance
only. There is nothing genetically different about any of the shades of brindle - just a matter of how many stripes the dog has. The opposite is "light" brindle - describing a dog that has very few brindle stripes. And in between all that, there is "golden" brindle, "fawn" brindle, "brown" brindle, "red" brindle, "mahogany" brindle, etc. These simply describe the shade of fawn ground colour, exactly the same as for differing shades of solid fawn boxers.
Since there is nothing genetically different about dark brindle dogs (including reverse/seal brindle) you cannot breed "for" it. A light brindle dog mated with a fawn might produce dark brindle puppies. Two dark brindles might produce a litter of fawn puppies. You probably (possibly) have a higher chance of producing dark brindle pups from dark brindle parents - but certainly not in any predictable manner.
You might like to note that although the US and Canadian boxer standards allow for "reverse" or "seal" brindle dogs, most (possibly all) other boxer standards round the world describes the colour as undesirable or even a named fault (same goes for light brindle).
2. "Seal" brindle is a term used in some parts of Canada to describe the same colour that is called "reverse" brindle in the US. Once again, it is a description of a shade of brindle only. Not a different or new colour of dog.
3. There is no such thing as a black boxer. It is a genetic impossibility for a boxer to be purebred and also black. The gene for solid black coat colour simply does not exist within the boxer gene pool. And that is something that 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 (labradors, for example).
There is more information about coat colour inheritance at www.boxerworld.com/coat_colour
And what to look out for in breeders advertising so-called black boxers on our FAQ page www.boxerworld.com/faq
You should also note the following from the rules:
"Black boxer" threads are not allowed: there are no black boxers because it is a genetic impossibility to have a pure bred black boxer. If you "see" one it is likely a cross breed or a "reverse brindle". We have enough of back yard breeders advertising them.