Be careful when using flitz or similar polishing compounds to polish blades that have hamon (don't remember if the ninja model has one or not). While it does not wear away material in such a way that it would remove bluing, it can burnish the surface enough to mask the hamon and make it less distinct or even invisible.
You can use it, but you should be careful if you want to preserve visible activity in the steel.
In traditional Japanese swords, you could use what was called uchiko, which is basically just the gathered powder from the last stones of the polishing process. It is screened through a little ball so large particles that could scratch do not get through. The uchiko ball is tapped on the surface of the blade, and the powder is then wiped off. It has a very very mild abrasive quality to it, though it may not be aggressive enough to remove certain things right away (takes time and repetition to more fully restore the blades)
Since you are dealing with a relatively inexpensive production sword, uchiko is probably not necessary, so use whatever is convenient.
