Kuroko

Kuroko (黒子, "black person"/"black clothes"^?) are stagehands in traditional Japanese theatre, who dress all in black.

In kabuki, the kuroko serve many of the same purposes as running crew. They move scenery and props on stage, aiding in scene changes and costume changes. They will also often play the role of animals, will-o-the-wisps, or other roles which are played not by an actor in full costume, but by holding a prop. Kuroko wear all black, head to toe, in order to imply that they are invisible and not part of the action onstage.

The convention of wearing black to imply that the wearer is invisible on stage is a central element in bunraku puppet theatre as well. Kuroko will wear white or blue in order to blend in with the background in a scene set, for example, in a snowstorm, or at sea, in which case they are referred to as "Yukigo" (雪子, snow person^?) or "Namigo" (波子, wave person^?) respectively. As this convention was extended to kabuki actors depicting stealthy ninja, historian Stephen Turnbull suggested that the stereotypical image of a ninja dressed all in black derived from kabuki. Real ninja, living prior to the advent of kabuki might have dressed in this way occasionally for night work, but certainly wouldn't have worn black at all times.

...
Wikipedia