First off, I am probably the closest person here to the obtaining the "right" to wear the black jacket as a fencing Master. I'm certified as a Provost at Arms from the SJSU Fencing Masters program, and am planning on taking my Maestro's exam in two years. I was the only person at WMAW who wore fencing whites for all four days. If anyone should be offended by other people wearing black, it by all rights should be me. That said, I find that I just can't get worked up about the issue.
Within the Classical Fencing world, it is understood that the black jacket is the prerogative of a Fencing Master. In the American Sport fencing world, black jackets are used by almost every coach regardless of certification level (this mainly due to the fact that coach's equipment is almost exclusively available in black). In some European countries, France in particular, certification as a fencing master is required to be a fencing coach, this means that all fencing masters get to wear black. As a rule, however, in the sport fencing world the black uniform -> fencing master rule is pretty much dead, and only preserved in the rule that no competition uniform may be black.
I personally do not wear a black uniform. At HEMA/WMA events I wear fencing whites for the entire course of the event. I do, however, own a black leather coach's jacket for use when giving lessons. That said, it's a personal choice as far as I am concerned. As a classical fencer, I would prefer that everyone wear white jackets, especially in the rapier/singlestick/broadsword/heavy saber (post 16th century) side of things, but given the many things that concern me about HEMA/WMA, this is probably at the bottom of my list.
That said, it is my experience that when judging point weapons (rapier, foil, epee, etc) and thinner cut weapons (modern saber, heavy Victorian saber, and singlestick) white jackets are considerably easier to detect touches on. I don't have a whole lot of experience judging longsword, so I won't offer an opinion there. I also think that, as a coach, it is easier to see student movement in white uniforms because the contrast lets you see a bit more of what's going on.
That said, black looks cool, and I coaching equipment (which is heavily used for HEMA kit) is pretty much only available in black. So, it's up to the practitioner.
Now for some history:
This rule is of relatively recent vintage. Fencing uniforms have gone through a number of changes through the 19th and 20th centuries, resulting in both black and white uniforms being used for competition. In the late 19th century, black uniforms and chalked tips were common in foil. The 1891 AFLA rules included "17. Each competitor must wear a dark fencing jacket, so that white chalk marks can be easily seen". There were also half black, half white uniforms, used to determine target area. http://www.museumofamericanfencing.com/site/index.php/fencing-history/early-afla-usfa/258-oconnor-tatham-nadal-van-zo-post-1891
shows both of these types. The description for that image includes reference to both "black cloth" and "purple velvet" uniforms being worn at this event.
Some 1893 comments on the black uniforms by a French fencing master: http://texasfencing.proboards.com/index.cgi?action=display&board=Misc&thread=516&page=1
In any event, these types of uniforms were phased out by the second Olympics, giving rise to the white uniforms we know today. I suspect that this has to do with the difficulty in judging using the black uniforms, from personal experience it is considerably more difficult to detect touches with point weapons with black uniforms (even with the colored tape). Taken with the comments above, I suspect that this was a big part of the switch. (That, or as in all things fencing, the French liked white and got their way)
I do not know when the "black for fencing master" rule was codified, and it's entirely possible that it predated the switch to black uniforms. (Previous to the 1890s, uniforms were most often white. All of the 19th century fencing manuals that I can think of show white uniforms, for example. ) That said, the classial white->fencer, black->master is not as hard and fast a rule as we might think.