Ninjas don’t wear black. They used to disguise themselves as civilians. Unlike ninjas in movies, the real guys were smart enough to know that wearing a black outfit with a face mask wasn’t the best strategy for blending in. Source
But this leaves out the really neat part! The reason we equate the above image with a ninja comes from Kabuki theatre. Within Kabuki theatre there’s a convention of having Kuroko (stage hands) dress in all black (with a full face covering) and move around among the costumed actors in full view, moving scenery, props and costumes. In a similar way, Bunraku puppeteers dress in all black, and only the lead puppeteer’s face would be uncovered. The audience knew to ignore these people and focus on the actors, and to only see that the scene was “magically” changing. So when a play called for a ninja assassin to jump out of nowhere and kill someone, the easiest way to create the surprise reveal was to disguise the ninja in the all black garb of the Kuroko and to remove the face covering and start acting at the last second. This would shock the audience, who were conditioned to not focus on them. Pretty cool, yeah?
WHAT THE FUCK I THOUGHT “STAGE NINJA” WAS JUST A CUTE THEATER TERM FOR CREW. THERE WERE LITERALLY GOD DAMN STAGE NINJASAS A FORMER STAGE NINJA I CAN CONFIRM THIS IS BOTH A THING AND AWESOME.
This is the best and most delightful piece of information.