Diabetes: black spots around the neck. Any way to treat this?

Yes. What you're explaining is acanthosis nigricans could be a sign of high blood sugar and diabetes (insulin resistance). Some medications like growth hormones or birth control pills can also cause acanthosis nigricans. With weight loss and control of diabetes and lowering blood sugar to normal levels (less than 100 fasting and less than 140 random), a lot of times the condition resolves over time.