Nick,

It will not be rust...not if it was not there beforehand...the blood in the meat causes this...I can see an area of "stain" in the photo...it looks like a form of "patina"...if you have a combination sponge/green nylon wire scrubber for pans...put washing up liquid on the blade and rub it with the green nylon under a hot tap...it will come off.

The amino acids in the blood make it react with the steel...happens on paunching rabbits as well...I carry a "scourer" as described above in my "Mess Tins" and use it with washing up liquid to clean the blade...the nylon type wire wool is less abrasive than the "steel" wire wool...it will do the trick!

Dried blood is always difficult to get off...left unchecked it will cause pitting on the edge...most of your marks are on the sharpened edge where there is naked steel which is rough...you would get this on a Combat finish or a LE finish. Using a leather strop on the edge and convexing it smoothes the roughness and does seem to help.