Blackheads are small black skin projections often found on the face especially on the nose and sometimes on the back. These black projections form a plug blocking the sebaceous glands of the skin. They are caused due to excessive production of '˜sebum' (an oily substance) from the sebaceous gland that slowly gets hardened when they mix with dead skin and bacteria. The sebum gets exposed to oxygen from the air and gets oxidised and turns black.