Arm dagger with blackened blade with unusual blade design ?

[katana]

I have an arm dagger which has a blackened blade. I am not certain as to why the blade is shaped like it is, the rust and pitting suggests that the this shape was done when manufactured ...or soon after.
The blade is 16cms...25cms overall. The handle is horn and ..I think..with reptilian skin on the sheath.
Please any comments or suggestions gratefully received...thankyou