Silver nitrate replenisher gone black

This is a fresh soultion of replenisher for my sensitizing bath. I was not careful and added the crystals using a steel spoon. Could this have caused the discoloration? I know it was a bad idea. It turened black as soon as I added the water into the beaker. The silver nitrate is a new batch, may be it is not good quality, bought through an auction on internet ( a bit greyer crystals).

Any opinions? I think I will filter it through cotton wool before use.

Done