Blackheads look as though dirt has become trapped in the skin.
That is not the main reason they are dark.
A blackhead is an open clogged pore.
Oil and dead skin create the plug
Like a pimple, a blackhead begins when sebum and dead skin cells collect inside a hair follicle.
The material forms a plug.
The key difference is what happens at the opening.
The pore stays open
In a blackhead, the top of the clog remains exposed to air.
That exposed material undergoes chemical changes, including oxidation.
Pigments and other compounds near the surface darken.
That creates the familiar black or brown appearance.
Why is it not just dirt?
If dirt were the main cause, washing harder would solve blackheads.
It usually does not.
The dark material is part of the clogged follicle itself.
That is why blackheads can return even on clean skin.
How is a whitehead different?
A whitehead forms when the clogged follicle is closed over by skin.
Because the plug is not exposed to air in the same way, it does not develop the same dark surface.
Are blackheads the same as pimples?
They are part of the same acne family.
Inflamed pimples involve more obvious redness, swelling, and immune response.
Blackheads are usually less inflamed.
Why do they often appear on the nose?
The nose contains many sebaceous glands and visible pores.
That makes clogged follicles there especially easy to notice.
A blackhead is not a pore full of dirt.
The dark color comes mainly from chemical changes at the open surface.
And that’s the short explanation.
Related explanations: Why do pimples form?, Why do scabs itch?, and Why does skin peel after a sunburn?