The black "uniform" comes from the Edo stage theater. Prop assistants and others who are supposed to be "unseen" in Kabuki, Bunraku, etc., wear all black uniforms to signify their "invisible" role. When Edo-period playwrights struggled on how to show their "invisible" assassin characters, they hit upon clothing them like the stagehands. The audience of the time KNEW that the character was "invisible", and understood the point. The common image carried over into film, unfortunately, and modern audiences frankly aren't as in tune with the conventions of the theater.
What DID "Ninja" wear? Anything that wouldn't attract attention. Peasant, traveling priest, low-level samurai, merchant--all these were perfect disguises for walking around the countryside, gathering information for your lord.
The black "uniform" comes from the Edo stage theater. Prop assistants and others who are supposed to be "unseen" in Kabuki, Bunraku, etc., wear all black uniforms to signify their "invisible" role. When Edo-period playwrights struggled on how to show their "invisible" assassin characters, they hit upon clothing them like the stagehands. The audience of the time KNEW that the character was "invisible", and understood the point. The common image carried over into film, unfortunately, and modern audiences frankly aren't as in tune with the conventions of the theater.
What DID "Ninja" wear? Anything that wouldn't attract attention. Peasant, traveling priest, low-level samurai, merchant--all these were perfect disguises for walking around the countryside, gathering information for your lord.