There has been multiple discussions that the DC should remain a static DC 14 instead of scaling with level. The idea is that you are getting an unlimited use poison, as bonus feat, and that scaling up the DC way overpowers the investment required to get it.
It's an Exalted feat, something that requires the character to maintain a high moral standard of behavior or lose.
Vile Feats, the opposite of Exalted feats, on the other hand are pretty much never going to be lost, unless someone goes all Darth Vader and attempts to redeem themselves, something Evil NPCS are likely never to do.
TLDR: Evil, unlimited use Posion, that scales in effectiveness, for the investment of a feat, requires you to be evil while using it is automatically an Evil act is probably a sub-optimal decision.
If you are intent on doing it, I would keep the DC a static DC 14, and make sure there is a prerequisite feat expenditure other than the feat for Touch of Black Ice.
Otherwise you just created a feat that would be a no brainer for EVERY evil character with a CON score of 13+ to take.