Bromden said:
Also you don't have to be a a ninja (or a genius) to dress in black for any kind of suspicious night activity. I don't think spies and assassins had exclusive rights to wear black clothing in feudal Japan.
...as well as there isn't any real historical source to back up that the shinobi even wore such a thing in the first place. The historicity of the 'ninja' is one big heap of trash that's got historical sources (which is very rare in the first place) mixed up with modern day fantasies and commercialized crap, that the truth is the "researchers" themselves are all confused about what is fact and what is fiction.
All we can deduce at this point with what limited information there exists, is that the historical 'ninja' were most likely a clan/band of information salesmen hired by local feudal lords under either a case-by-case contract, or a set service term. All the rest of the tales about how they were superhuman combatants, or studied a zillion different arts to become a super-spy/assassin of sorts, is basically the Far-Eastern equivalent of Codenumber 007, James Bond. Notice how James Bond does exactly everything no modern day subterfuge agent would ever do.
Just as in real life, the best and most formiddable spies are someone nobody would ever suspect -- teachers, housewives, students, everyday-man who just happens to work near sensitive material or know who handles them, it is most likely ninjas were such as well.
Besides, such spies/information salesmen existed in any region of the world where there was political/military tension, at any given era of history. Recorded documents of spy networks and its use date back to even to the ancient Roman days... in some cases even further back in time.
Basically, it is safe to assume that any info about the ninja that involves scaling walls, wearing black, throwing stars/shuriken, assassinating people and infiltrating guarded places, is simply fictional crap created some time between early 19th~mid 20th century, where romanticized action novels became greatly popular among the masses. Upon that tradition, modern novelists such as the legendary Yamada Futaro (often cited as the 'father of the modern ninja fiction') or the renowned Shiba Ryotaro, have reinvented the image of the ninja through their most famous works. Most REAL researchers (not those self-described 'researchers') credit those two authors as the beginning of the "ninja" fantasy-fictional works.
[Edit] Not to mention that in real life, infiltrating guarded positions are next to impossible. The whole reason the castle/fort walls were plastered in white in first place, was for the guards to immediately notice if there was somebody near it. Also, most ninja fiction treat guardsmen as imbeciles and morons, despite the fact that in reality guards and their captains were most usually the most seasoned of soldiers hand-picked in recognition of their skill and dedication.
Even with superior technology and training, modern day military infiltration units require weeks of observation and planning, coupled with insurmountable advantages such as satellite imagery, to be able to sneak in and get the job done. It needs all sorts of total support both near and away from the scene to work.
[EDIT2] The era during which the ninja supposedly existed, were the Warring States period of Japan -- absolutely the most paranoid years of Japan. There are countless town records of how the townsfolk accused wandering travellers, merchants, monks, nomads, ronin, etc etc.. as being spies, and simply the local watchmen would beat them to death and throw their bodies away. Any outsider would immediately be viewed with extremely suspicious eyes, with local police forces trying to keep tabs on them. Even just lurking into the local tavern to get drinks and start asking questions, could be enough for someone to come and arrest you in those days.
[EDIT3] The only form of successful assassinations in real life were usually by solicitating traitors from the inside. Most assassinations aren't even planned well in the first place, usually coming from disgruntled retainers or employees. The historically notable cases of assassinations from outside agents are usually concentrated on the medieval Hashishins of the Old Man of the Mountains -- and usually their rate of success stems from the fact that the assassins simply gave up trying to safely return from the attempt in the first place -- effectively making them the medieval equivalent of the suicide bomber.