Regarding cleaning the knife, I fully agree with Detlef.
What you think is patina, is actually rust. While black rust is less active (but still active) and may contribute to the character of a blade, brown rust does nothing but damage the blade. And it is precisely brown rust what you have on your blade.
Moreover, it is as you said a 19 century blade (best case scenario) but it can also be from early 20 cent, so not so old. While is I consider acceptable to have some black rust on a 16-18 century blade, I find it less acceptable on such a recent blade.
But this is my personal oppinion. It is your blade abd your call.
