Raise Hell and Mutts

The depiction of ninjas as dressed all in black comes from traditional Japanese theatre.  Actual historical ninjas didn’t dress in black because it’s conspicuous as hell in the daytime and even at night in the dark a person dressed in solid black tends to stand out; dark grey or blue is better for hiding in shadows.  Usually they just wore ordinary, like, people clothes which are far better for blending into your surroundings in than a specialised professional costume.

BUT YOU KNOW WHO DID DRESS ALL IN BLACK LIKE THAT

the stage crew in a theatre

and it was a generally accepted convention that the black-clad stagehands were invisible, so they could be on stage at the same time as the actors and move things around and the audience would just mentally CG them out

but then one day because a director was a GENIUS, during an otherwise normal performance of a play, suddenly a stagehand stepped forward, assassinated one of the main characters and then melted back into the background

THEY WERE A NINJA

AND THE AUDIENCE LOST THEIR MINDS BECAUSE IT WAS AMAZING

and eventually it lost its mind-blow value because after a while everyone had seen a play like that, so although the “stagehands wear black and are invisible” convention continued, the new “ninjas wear black and are invisible until they choose to strike” convention became established, and from then on fictional ninjas have just worn black because it looks so cool.