You should clean it but clean it with a non harse abrasive cleaner. It works far better when cleaned, I had to clean my mothers Silverado today and it worked alot better. The reason it gets dark is because its a DC device which works on the basis of 'electrolysis'. This is the way they plate products in metals like Gold, Silver and Chromium. This is why when you make Colloidal Silver the one probe gets dirty whilst the other is ok and you have to clean it off. A large amount of the Silver or Copper driven by the zapper into the body is being collected back onto the other electrode which if i remember right is always the Positive electrode.

Hope this information helps!