A blackhead or comedone is caused by excess oil that mixes with dead skin cells forming a thick, sticky substance. A blackhead or an open comedone looks like a large, dark pore. The black in a blackhead isn't dirt. It's dried oil and the skin cells that gather in the opening of hair follicles. A whitehead or closed comedone, has skin growing over its opening and looks like a small, hard white bump. You can often get blackheads and whiteheads from using oil-based makeup and hair-care products. A lot of sweating and humidity also causes them.