Well, I feel that if it came from an R Rated Dressage Judge, then there must be something to it.
White gloves draw more attention to the Judges eye. Of course the Judge will beable to decipher whether your hands are educated or not with black gloves on, but black gloves do not accentuate it.
That's like - don't put white tape on your horses braids if you don't want the Judge to have their attention drawn to your horses neck and "head set"
If a Judge tells me that they prefer black gloves on lower level riders, then I'm going to go with what she told me.