The true blue brindle that does appear in other breeds is a result of the dog carrying both the gene for brindle marking and a gene for color dilution. The color that is diluted to get the blue/grey is black. Do an internet search for "blue doberman" and you'll see lovely examples of dilution in action.
That being said, Boxers don't even carry the gene for the color black. The only way that a Boxer could be black is if it were the direct result of a cross-breeding or the result of a cross-breeding generations back.
NOTE** I'm talking geneotypically here (what is actually in the genes) not phenotypically (how the dog looks). People do claim to breed purebred black Boxers but if they are in fact not the result of cross-breeding then they are just reverse brindles. Reverse brindles are dogs that are a dark brindle but the brindle marking is such that the darkest, blackest portion of the brindling is so predominant as to give the appearance that the dog is completely black (with or without white flash).
So you might ask yourself "If you can have a reverse brindle that looks black, couldn't you have a purebred Boxer that was brindle that also has the dilution gene and end up with a 'blue brindle'?" Nope. Boxers aren't supposed to carry the dilution gene so you'll never see any true (phenotypically speaking) "blue brindle" Boxers. So if it's in there, it had to have come from an outcrossing at some point.
Again, all this is genotypically speaking and the genotype of coat color is a really confusing subject. I certainly wouldn't sell myself as an expert or anything--not even close! But I'll give example of how things can get really muddy. How many of you have seen a cream-colored French Bulldog? The color is really popular. The problem is, apparently cream isn't a real color in Frenchies at all, or so I've read. In genotypic terms it is in fact a very dilute fawn without a mask. Here's the rub though--Frenchies supposedly don't actually carry a dilution gene! So where the heck did the cream come from? And does that mean that all those cream-colored Frenchies are in fact not Frenchies at all? They sure look Frenchie to me! Somehow a dilution gene has crept into some Frenchie bloodlines but according to the standard, it shouldn't be there.
So the layman in me thinks that if it's possible that it can happen in one breed, perhaps it's possible in another. Maybe those "blue brindle Boxers" are in fact the result of a dilution gene being introduced somewhere, somehow. Whether it was through accidental means or unscrupulous means, who's to say. Either way, to proport that this is some great new breed color or is rare and therefore special and desirable is what is truely unethical. Especially since dilutes in other breeds can tend to be more prone to skin issues.
If you want to be completely confused by coat color genetics and humbled by the amount of information you don't know, consider reading this blog entry:
http://musingsofabiologistanddoglov...1/10/guess-genotype-breakdown-of-alleles.html
or this article:
Coat Color Genetics
