If the "tarnish" you see is blue (could it be greenish?) it's copper and/or the copper component of brass. If the snake has any brass gizmos on it, soaking it in solvent will continue to eat the copper/brass.
I don't use snakes but everyone I know that does tends to keep them dry and use them just as a dry pull through after a day at the range, just to pull out the big bits of crud.
In other words, I'm thinking that your soaking the thing in solvent may be what's causing the blue discolor. Or it may be dissolved copper/brass from the cleaning itself.
Brake parts cleaner cleans most anything. Wear eyes and gloves to use that high pressure stuff.