I have black circle across the spinal cord as well as to sides of the spinal cord (no itching).Could you please tell me how to prevent this black?

See your doctor. Black can be from external or internal pigment. Your body can produce things which may be benign such as a seborrheic keratosis (my best guess without looking at it in person) or may be a melanoma or even another issue. You should see your doctor to diagnose exactly what it is. Then, you can discuss ways to prevent it if that is appropriate, or your doctor may have another recommendation.