Frostbite can become gangrenous in bad cases, which is where you would see necrosis like this.
So this could just be some horrid case of frostbite.
I love it ^^
Looks good, like the transitional aspect of it... it doesn't just stop, which can be a bit of an issue with a large area like the arm.
BTW, found the best thing to color gelatin is food coloring.
Again, nice work....fave'd