A ninja is a figure in a head-to-toe black bodysuit and mask, melting into the night, only the eyes showing. Every film, game, and Halloween costume agrees. Black equals stealth equals ninja. It is one of the most instantly recognisable looks on the planet.
Real shinobi, the spies and irregular fighters of feudal Japan, did not run around in matching black pyjamas, because the whole point of espionage is to not look like a professional sneak. They dressed as what they needed to be: farmers, monks, merchants, travellers, anyone who would not draw a second glance. The all-black costume comes from kabuki theatre, where stagehands called 'kuroko' wore black to signal to the audience 'ignore me, I am invisible'. Playwrights used the same convention to reveal a hidden assassin on stage, and audiences learned to read black-clad equals unseen. Film and manga inherited it, and a theatrical shorthand became the 'historical' uniform. The deadliest disguise was never a ninja suit. It was looking thoroughly, forgettably normal.