http://budk.com/images/500/BK385.jpg
THIS is the style classically attributed to the Ninja. double-edged, and only curving at the tip.
http://budk.com/images/500/BK277.jpg
this is the type i was referring to, belonging to the group of ALLEDGED "ninja"
http://budk.com/images/500/UC1258.jpg
this is the sort of blade i assume you mean when you say ninja blade (slightly curved, straight edge...ignore the handle)...it is a SAMURAI weapon, not a ninja one
oooo...i just found a katana similar to one i tried to train with as a child...a standard samurai katana, but with both the leading and trailing edges sharpened....i stopped trying when i cut my ear on the backswing
EDIT: i also just realized that that second style was also used in the movie The Last Samurai....while it was true that the group using it were supposed to be ninja, the sword itself was not used by the ninja...in fact, the swords being used were clearly not even an authentic style, which i found rather funny.