The difference between whiteheads and blackheads relate to variations in the skin follicles and whether they are closed off from the surface or not
Whiteheads are white to the eye, and occur when the follicle becomes closed off at the surface of the skin. The sebum, dead cells and bacteria therefore build up underneath the follicle inside the pore, causing the white coloration
Blackheads are black to the eye and form when the follicle becomes clogged. This material is not blocked off from the surface, but the clogged build up causes the edges of the follicle to expand, causing the pore to appear larger. The dark coloration is the visible sebum, dead cells and bacteria in the pore, not dirt as is commonly held