There have been rumors about "blue boxers" in Estonia and Russia during the last ten years, and Finnish boxer breeders and judges have seen them there in dog shows. These blue boxers are said to have normal yellow/fawn and white areas, but the black pigment on face and on brindle stripes is not black but dark gray. Nose leather is black and eye color is normal. Gray pups can be told from normally colored ones immediately after birth, if breeder knows to look for them. There have not been any reported health problems with this color. I haven't yet been able to see such a dog or even a photo, but as I have now discussed with an Estonian breeder, Lilian Pajuland, I can present some guesses about the genetics behind that color.
As these blue dogs can appear in litters where both parents have normal black pigment, it has to be a recessive trait. I don't have any breeding statistics, but I guess that it is inherited as simple recessive. Everything else but that black nose leather seems to suggest that these dogs are dd blue ("Maltese") dilutes, like blue Danes and blue whippets. It is possible that the nose leather is dark slate gray anyway, but so dark that it can't be told from true black nose - or then this blue gene in boxers is another gene, not related to d.
Other possibilities for gray or "blue" color in dogs include:
As you can see, I rule out all the other genes for blue color and so give my vote to d diluting gene. If a test breeding between two blue boxers gives just blue pups, it proves that this gene is recessive and if a blue boxer was bred to a blue dog of another breed, like a Dane, it would be easy to see whether this gene is the normal blue dilute or a different one (all blue litter = the same gene, all normally black-pigmented pups and no blue = another gene).
Estonian and Russian boxer population had been almost totally cut away from the rest of the world for decades, and during that time a rare gene can easily get more common in a smallish, closed population. Estonian boxers have been imported to Finland during the last seven years, and some of them have been used for breeding. This far they have been bred only to Finnish boxers, and blue pups have probably not yet been born here, but after a generation or two there will be litters which have Estonian dogs on both sides of the pedigree, and blue pups might start popping up.
Or perhaps we have had blues already, but breeders have failed to recognize them. A dark brindle boxer has so much black pigment that it is easy to see when it all has turned blue, but a masked fawn or light brindle might pass just as "lighter than normal" pup and probably be placed in a pet home.
There are at least three possibilities for the origin of this blue gene in boxers:
1. It has been in the boxer population from the very beginning of the breed, but it had been very rare and either had disappeared on its own or bred out from most boxer populations of the world, but happened to survive in this population.
2. A mutation has occurred and one normal D gene for black pigment has turned into d gene for diluted blue pigment. Mutations do happen, but they are extremely rare. If it can be proven that this gene for blue boxers isn't d but something else, then I believe that a mutation has occurred in Estonian boxers.
3. If the gene is the same as the blue gene for blue Danes and blue Neapolitan mastiffs, then it might have come from a cross between boxer and one of these breeds. If this theory is true, the cross (perhaps accidental) has probably happened decades ago, and modern Estonian boxers can't be considered as mutts more than any other boxers. This is the most probable theory, at least in my mind.
Let's first look at what the boxer standard says about this subject.
This is taken from the original German standard: "Schwarze Maske. Die gestromte Varietät hat auf gelbem Grund ... dunkle oder schwarze ... Streifen." (My unofficial translation: "Black mask. The striped form has dark or black stripes on yellow background.")
American (AKC) standard says something like this: "The brindle ... clearly defined black stripes. On the face ... essential black mask."
And these parts are from the Canadian CKC standard: "The brindle coat ... clearly defined dark stripes on a fawn background. On the face ... essential black mask. ... The nose is broad and black."
As you can see, the standard calls for black mask. The stripes don't have to be black (except in the US) as long as they are dark, but that dark slate gray face instead of deep black mask still means that blue color is a fault and blue dogs should be punished in a show ring. If the standard is changed or at least interpreted so that it is enough that the mask is dark, blue color is ok.
Blue color can often be connected with a skin condition, color dilution alopaecia or "blue dog syndrome" (CDA), which causes hair loss. As this gene affects many blue dogs, especially in short-coated breeds like Dobermanns and Danes, it might cause problems to blue boxers too. I suggest that any dogs with blue pigment be watched closely for any signs of skin problems, and if blues have more problems than normally colored boxers, it might be best to try to eliminate blues by keeping record of blue carriers and avoiding to breed them to each others, and by neutering all blue pups. It is very difficult to get totally rid of the blue gene itself, as it is recessive.