First of all I want to reassure you, if your jewelry blackens it is completely normal. Silver jewelry can darken over time.
It's a natural phenomenon and neither you nor the jewelry brand had anything to do with it. No need to contact the brand or throw your jewelry in the trash, you just need to know how to clean silver jewelry to make it look as good as the first day.
There are a whole bunch of grandma's tips for cleaning silver jewelry.
1st method to clean silver jewelry with baking soda
Mix a tablespoon of baking soda in a bowl of hot water and soak the jewelry in it for a few minutes. Then rinse the jewelry with clear water and dry it with a soft cloth.
2nd method: Clean silver jewelry with lemon
Use a lemon previously cut in half and press it on a soft cloth. Rub your jewel well, then rinse it without forgetting to dry it well.
3rd method: Clean your silver jewelry with white vinegar
Put your jewel to soak in a glass of white vinegar for about 2 hours. Rinse it with lukewarm water and dry it with a soft cloth or chamois leather.
What not to do to clean your jewelry
Clean your silver jewelry with a toothbrush with hard bristles: The toothbrush can scratch your jewelry. You will be extremely disappointed not to be able to wear them afterwards. Use a soft cloth or chamois instead.
Use chemicals and clean your jewelry: they are abrasive and strip the jewelry. They take away their shine and damage stones and pearls. Prefer baking soda, lemon or coarse salt! Also, do not put your silver jewelery in the dishwasher, as this can damage the surface and the stones.
Sleep with your jewelry: At night we move a lot, and the risk would be to break a chain, lose a pearl or a mother-of-pearl flower or even lose a clasp during your sleep