Katana cleaning

[Dragoncat089]

A picture of it:
https://drive.google.com/file/d/0B-JwwJXPx2rIYmVVRjYtR2RUQUtmNkFCa1g3anVINnRFU1l3/view?usp=drivesdk

I recently got a katana from my friend. They had it in storage for a while and when we got it out, it had some weird black stuff on it. I cleaned and oiled it, but it didn't come off. Any advice?

 

[adamlau]

Looks like the blade coating was improperly sprayed/baked or it was not applied thick enough to fill the voids of the underlying steel.