Every month for a week my nails are stained black which looks like dirt under the nails and a stain over the nail. What diagnosis would apply to that? Diabeti

Doctor can evaluate. A primary care doctor or a dermatologist can evaluate to get a complete history, including all the things the patient touched during the 2 days before her nails got stained. The staining is probably from an external substance, because it would be very odd for a person's nails to repeatedly be stained 1 week, normal for 2-3 weeks, stained 1 week, normal again for 2-3 weeks, stained again, etc...