Where does black line appear?

I don't think I can answer that cuz' I do not know what black line you're talking about

If you are asking about pregnancy, then in that case, this is also called Linea Nigra. One of the most common sign of pregnancy. It runs up the abdomen between the pubic area and bellybutton, or even higher. It is usually about. 25-.5 inch wide, growing darker as the pregnancy advances.