Hi, While cutting up a deer that had been hanging in the shed for 3 days I noticed than the old blood was etching the blade on my carbon steel boning knife and changing the colour of the steel from a bright finish to a mottled blue/grey & black - not unlike the worn case colour finish on some of my Winchesters SS. I tried to remove this colour with some OOO steel wool but it had penetrated the steel quite deeply. Actually after this polish it looked quite nice. 
What's going on here? Anyone else seen or used  this process?I assume the blood is  slightly more acidic due to breakdown processes. More importantly could this be an easy method for recreating case colours on actions rather than using the old propane torch & perma blue method. It would certainly look better tha a straight reblue job.
I'm going to save some blood from the next deer (is species important??) and run some tests on an old low wall & Stevens 44 frame.
Spud