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.
Yes, as stated, it would appear that your solvent is reacting with the brass or copper. Many of us have leaned this the hard way and I have seen brushed actually been eaten away. I could be wrong by I believe that it is the ammonia in the solvents. I see this typically when I use Hoppes #9 or similar solvents. After using these, I try to rinse with mineral spirits. My primary go-to solvent, is mineral spirits and go from there.
Be Safe !!!