When did it become conventional for stagehands to wear black clothing backstage?

Answer 1

that trick was used as far back as there were stages with artificial lighting it just made too much sense to have then wear anything else but black to cut down on being seen ....for the same reason that you walk slowly so you air wave from movement will billow the curtains or scrims as little as possible when you have to go across that close to the stage... newer stages have quick pass areas back behind the action so your movement will not be seen, black clothing or not