Is a black ring around the neck a sign of diabetes?

Maybe. A dark, velvety patch of skin on the back of the neck and underarms may be 'acanthosis nigricans, ' which can be a sign of Insulin resistance, which can be seen in ppl at high risk of developing diabetes, pre-diabetes, or diabetes.
Ring. It could be. There is a condition wher there is increased pigmentation called acanthodians nigerians that can go along with type 2 diabetes. Some fungal infections can also have a similar appearance.