Ninjas were masters of disguise

The ninja is the legendary figure feared throughout history for their agility, stealth and above all superior fighting skills.

They are always depicted in an entirely black outfit  with just a small slit for their eyes. They usually brandish various weapons such as one or two swords, a set of nunchaku or even the dreaded "flying star" shurikens.

There's a big problem with this picture, however. Ninjas did exist, and are well documented as a kind of mercenary force performing special missions where the army or Samurai were not best suited, for example espionage.To achieve this, they were renowned masters of disguise, often blending in perfectly with ordinary people, workers or government officials. You see the problem here? Any kind of instantly recognisable outfit would make this role completely useless. The impression given throughout history is wrong.

Ninja: "To steal away, to hide"

The origin of the Ninja as a description of the role can be traced to the 15th century, where often those from the lower classes were recruited because they were easier to pay to carry out what was seen as the less savoury missions required. Often the role itself was kept secret, so even those living amongst them daily in ordinary roles would be unaware of their special activities. They were the early forms of spies as seen today in the modern James Bond movies.

Ninjas were largely freelancers, or mercenaries. Feudal Japanese lords would hire them on demand, but later ended up keeping some permanently in their ranks as an early form of security guard. The government also ended up employing some, since they made excellent bodyguards. When times were lean, they also sometimes used their skills to turn to crime, robbing highways and such.

In 2017  Mie University founded a research unit specifically to study the Ninja. Students attend a rigorous 2 year regime, including fitness disciplines, alongside their ordinary roles in real life, naturally.

The reason for the glaring historical discrepancy regarding the outfit is largely down to poetic license. No doubt, when forced to fight the anonymity of an all-black outfit which also covered the face would prove useful, but it was by no means a "uniform" the same way a spy today would never be identified by just what they wore. This isn't really a regular Mandela Effect either, nor is it one of the more obscure MMDE's because there are so many references to the all-black outfit today. Or perhaps it's so well disguised that we only think that's the case?