So you purist are telling me that back 200 years ago, they had 100% pure blooded boxers or did they breed other breeds together to come up with the boxer standard?
No and no.
First of all, the Boxer as a breed isn't 200 years old; however from about 170-120 years ago the Brabanter Bullenbeiser was selectively bred (and sometimes crossed with English Bulldog) to maintain and improve on the functional abilities desired at the time (in this case, an all-around dog - guard, escort, family dog, butcher's dog, hunting dog, etc.). In the late 19th century a group of fanciers got together and wrote a description of the ideal dog of this type, and formed a Club of like-minded people who valued the traits of the dog and wanted to protect it from negative change. They didn't just throw two breeds together as the 'doodle/mix' breeders do now; they took two established breeds of similar type and function, and made discriminate crosses to improve and enhance existing traits.
If so, there is no breed that is 100% pure, they just started breeding dogs with like traits after the standard was made.
No, in most cases breed standards (and the concept of "breeds") were created after the dogs had been purposely developed for their given function and were breeding true (consistent type) for generations.
I think it would be nice for someone to throw the black gene in there while maintaining the boxers looks and personality,
Run down to the shelter, or check petfinder.com - there are any number of Boxer mixes available, many crossed with black dogs, and Boxer mixes often are mostly Boxer-ish in look and personality. (As much so as you'd get from most "black Boxer" breeders, at any rate.)
It might knock off some health problems that boxers have and produce a better quality dog.
How so? The most likely breeds to cross with a Boxer for a black coat color without losing much type have many of the same genetic health issues that Boxers have - and a few, like PRA, that they don't. So you're doubling up on existing problems in the breed, and adding new ones as well.
Thats just my $0.02, but I know some people take the paperwork a little too far.
It's not remotely about the paperwork. It's about preserving the integrity in conformation, temperament, and health of the breed we love; it's about ensuring that people who want a Boxer because of their unique traits get the dog they want, with the best chances of a long and healthy life. I realize you're simply asking a hypothetical, but "black Boxers" are the latest money-making scam, with unknowing buyers paying $1500-2500 for these "rare-colored" mixed breeds who come from unknown backgrounds with a high risk of genetic health or temperament problems. It's difficult to speculate on "what if" the color existed, because of the ever-increasing number of unscrupulous breeders who are capitalizing on just such speculation. (The purple with green polka dots speculation is much more effective in that regard, since I can't think of any way one could actually breed such an animal!) If these dogs were sold as "black Boxer mixes" there would be far less cause for concern - but of course, the breeders couldn't get the exorbitant prices they're getting, either.