I can answer that question for you. Your silver is turning dark because it generates too much current, and thereby creates Silver MUD which is extremely large particles of silver.
It is doing this because there is no current control going to the electrodes. A quality generator MUST limit current to the electrodes to prevent making Silver MUD,
Preferably the current should be limited to not more than 2 milliamps. Just my opinion as I'm sure I will get some Flak over this. I prefer to take longer to generate silver to insure tiny particle size.
I don't think they make the Pocket Silver Puppy anymore, do they?