The black line known as dariers line is usually seen in people who have frequent cough and cold or those with a history of Asthama in family.
It is not a permanent thing but you have to make sure you dnt rub your nose frequently.
Next Steps
Lightening creams containing kojic acid or glycolic acid at night helps lighten it. But the faster option is to get a laser(q switched Nd:yag laser done
Health Tips
Use a sunscreen every day morning to avoid it from further darkening.