The name shikomi-zue is actually the name of a type of mounting; the sword blade was placed in a cane-like mounting (tsue), to conceal the fact that it was a sword. These mountings are not to be confused with the shirasaya (白鞘, "white scabbard"), which were just plain wooden mountings with no decoration other than (sometimes) a short description of the contents.
Some shikomizue also concealed metsubushi, chains, hooks, and many other things. Shikomizue could be carried in public without arousing suspicion, making them perfect tools for shinobi.
In Shinobido Edit
During Revenge of Zen's final battle and after the first phase, Shu will reveal a hidden sword inside his staff, which he uses to great efficiency with his pyrokinesis in a style similar to Zen and Nagi. Given the usage, shape and lenght of the blade, this sword is funcionally identical to a ninjato.