Genuine 925 sterling silver contains copper for strength which causes it to tarnish naturally when exposed to sulfur in the air sweat and humidity. Tarnishing actually proves the metal is real unlike fake plated jewelry that peels over time. Daily habits like spraying perfume applying lotion and storing silver on open trays or in humid bathrooms speed up surface oxidation.
You can easily protect your silver by putting it on last after applying makeup wiping it down after use and storing it in airtight bags. If your jewelry turns dull or black wash it with warm water and mild dish soap scrub it gently with a soft toothbrush and dry it completely before polishing it with a soft cloth to bring back its original shine.