LonBlue,
"The black" is not part of home plate. In fact, many plates that you buy have no black border at all. The purpose of the black border is simply to provide stark contrast at the edge of the white plate, as an aid to the umpire.
The size of the plate is specified in the rules. The white area is the area specified, the black area is beyond the specified area. So I guess you could say, the rule book indirectly says the black isn't part of the plate when it specifies the plate's dimensions. The rule book doesn't address the black specifically, because the black isn't even a part of the official field.
In amateur baseball, you get guys who ump for second incomes or as a hobby. They are not the very best and you have to expect some shortcomings, just as you expect high school players to boot more ground balls than MLB players.
What I don't understand is the MLB approach. An umpire who refuses to call the strike zone per the rule book should simply be told, "Either you follow the rules as written or you find another career." And then you need to fire a few until the rest get the message. Umps are to enforce the rules, not to make them up as they please. And consistency is not good enough, if you are consistently bad.
Eric Gregg was a well publicized example, but he wasn't exactly fired. He was part of the walkout/lockout and he miscalculated, because when many other umps were taken back, he was not. That's close to a firing but not quite. A better approach would have been to fire him immediately after the 1997 LCS where he was so very awful and became the issue in a game that ultimately determined a World Series champion. Sure, there would've been a union grievance proceeding, and maybe he would've won reinstatement in the end with some sort of probationary status, but with the video evidence they had, MLB should've tried to put its foot down right then and there.
They really still haven't put their foot down.