Dandruff is common in cats of many coat colors, but very noticeable in black cats. My now deceased black longhair had
dandruff at times,
and I treated that with brushing/combing, which he was thrilled with, and a high-quality diet, which he was less thrilled with. A bit of flax oil on his food seemed to help, as did a "pet shampoo spray" I found in a pet store, which moisturized his fur and skin without the hassle of giving him a bath.