Distilled water is definitely very corrosive. Given time it will do short work of the nickel plating on your blocks. It will also darken brass and copper parts
I believe that's what you're seeing, water slowly incorporating nickel and copper ions.
You need to use a corrosion inhibitor of some sort to avoid having that happen. The problem with Cryofuel is it uses another method of corrosion prevention that basically gives the water higher conductivity. It's totally fine for the loop, but just don't work with the AC sensor.
So, DP ultra it is, and maybe don't wait too much.
If i were you i'd drain the loop of contaminated water, and refill with the premix to start fresh.
At work (semiconductor fab), all the cooling circuit is purified water (not DI but close). Only old machines still have brass quickconnects and copper circuits. They clog and are flat out black inside, because of corrosion. All the rest has to be stainless steel because of its corrosive nature.