I've been reading a lot of (mostly BYB) breeders' websites and most of them have a story about how black boxers are as old as the breed itself. They all refer to an account by a girl in the early 1900's (i'm not sure who).
That was Friederun von Miram-Stockmann, widely considered the "mother of the breed".

What those breeders neglect to mention is that Frau Stockmann notes - several times - that the black coloring was eliminated from the breed shortly after it first appeared.
All that I was able to gather from this passage was that the first "black boxer" was bred by one gentleman who admitted to an accidental Schnauzer impregnation. Since that time, he apparently let the "black boxer" die out.
Right - the color came from a Schnauzer cross, and was never present in the purebred population. The German Boxer Club did not approve of the color, and made it a disqualification in the mid-1920s. Since these dogs were then unable to be bred (and had been unable to be shown prior to the DQ), the color quickly disappeared from the breed. (So the question, which none of the 'black Boxer' breeders I've asked have ever answered, is from what breed(s) did the color come in this latest fad?)
This woman had since attempted to find a "black boxer" and found one close, but it wasn't close enough... she she painted the dog with black shoe polish. Her mistake was seen on her dress after a show. I couldn't follow anything beyond that showed a real black boxer.
It really was a black dog, but the color was not as deep as she wanted on his flanks, so she used the shoe polish on him.

But I've seen show homes that do all the necessary health testing show very black boxers. Yes, there was a sparse fawn bit showing through on the reputable dogs... but why would they advertise their dogs as black?
Yikes, do they? The responsible breeders I know who have very dark dogs either advertise them as "reverse brindle" or, sometimes, "black brindle". I haven't seen one that actually advertises them as solid black.
Also, I haven't been able to find a direct breeder of import Boxers claim to have a "black Boxer" despite a very dark pup. Is this not a world wide, but rather an American, phenomenon?
It is probably mostly an American thing, because a) we don't have strict rules regarding which dogs can be bred and b) we have a society that is very keen to own the "rare" and "special" - hence the popularity of "designer dogs".