Hi, I did some forcing on my two GB's as well. With mustard, my own blood (when I cut myself accidentally) and Birchwood gun blue (see picture). That last option worked well, but by using the knife the effect wears off. Since there is a death-head on the package of the Birchwood I don't wanna use it for foodprep anymore, so I removed the patina and stopped using it.
I never left a Spydie overnight in mustard, a patato etc. I learned my lesson when leaving a lemon on a antique (Gunto) japanse katana for a few hours. Ouch....it left a permanent mark.....
Beware of sudden popping up black pits. Oil that M4 once in while, especially when you do forcing patina's!
Even in times of peace a gentleman still keeps a sword by his side