It's due to the metal oxidising. If you really can't bear to throw them away, try painting the inside with clear nail polish. It will eventually chip off but will solve the problem temporarily.
I wouldnt jump to any conclusions here. I remember hearing something years ago about why some people had this happen and it was to do with the person rather than the jewellery. You will need to do more research. A quick look on google and I found a suggestion it is an indication of iron deficiency.
Nothing at all to do with them not being real gold/silver. It is something to do with a chemical reaction in your body,acidic I think. Certain medicines etc can do this but it is usually temporary it eventually goes away.