Ninjutsu was the 'Art of Stealth & Concealment'. the black garb was merely for reconnaisance, information-gathering & assassination missions at night. Otherwise, they disguised themselves as anyone, from mere farmers, to merchants, to samurai. And of course, one way for Kunoichi (female ninja) to infiltrate was as a courtesan. They could gain info whilst at court of a warlord/shogun, even kill whilst having sex (the perfect way to get to your target).