It's possible the
word "black" was banned rather than "dragoon," because it might remotely be considered a racial slur in some way, shape, or form, but I have no idea what permutations you tried. On the plus side, apparently any other color that is commonly expressed as a human skin color is
also banned (sorry White Knights).
Hopefully color words were reserved for purposes of lore but part of me suspects that might not be the case. With this sort of pathological political correctness running rampant, if it is indeed such a thing, Blizzard can go suck donkey nards.
"Dragoon" may be reserved for lore reasons as well, now that I think about it. At this rate, we may as well have a list of "OK" words to select from. Lame.