Sebaceous gland is the main source which secretes the oily matter called sebum. You might get these blackheads on your nose when a dust particle enters on your skin which blocks off the sebum that should have been secreted outside the skin. With this barricade, the oil as well as the dust particle will coagulate and is formed into blackheads.
When there is too much production of oil from the sebaceous glands, bacteria will grew in multiple numbers causing inflammation in the form of blackness. If nose blackheads are present, one thing you must not do is pressing or crushing your nose.