Black dirt at the root of the dog's hair may be the feces of fleas and mites, or it may be blackheads. You need to check whether there is itching, scabs, dander, etc. on the dog. If so, you need to help the dog. Deworm or medicated bath; if it is blackheads, you can give the dog a bath, then adjust the diet, feed some lighter food, and it will get better after a period of time.