Also known as: 黒衣, 黒子装束, 黒子(歌舞伎), 黒衣(歌舞伎)
Traditional stagehands in Kabuki, or Japanese theater. Dressed in all black to blend in while manipulating props and assisting actors, with the audience ignoring them as part of the suspension of disbelief.
The stereotypical ninja look also comes from this, as ninjas on stage are sometimes dressed as kuroko only for them to suddenly "reveal" themselves by assassinating a character.