by black marks do you mean blackheads?
If yes fill a bowl with hot water soak a face cloth in and place on your nose keep the cloth as hot as possible and do for at least 10 minutes rub the cloth around the nose for a few minutes then place a cold wet cloth on your nose for a few minutes.
the first process with the hot cloth will open the pores rubbing your nose will remove the dirt and the cold wet cloth will close the pores, resist the urge to squeeze a week of this treatment once per day should cure the problem