What causes a massive blackhead to form?

A blackhead refers to something scientifically known as a 'comedone'.

Comedones are formed as a result of the blockade of the duct of something known as the ‘pilosebaceous unit'. This entity consists of a hair follicle and attached to it a structure which is known as the 'sebaceous gland.’ This gland is the oil producing factory of our skin and this oil production is in part controlled by androgens or the male hormones, which in a small quantity are also present in females making comedones and as a result acne vulgaris (pimples) a universal problem.

Now to understand this apparently complex description, just think of this as small oil wells lying beneath your skin and opening on to the surface via a pipe.

This oily substance is known as 'sebum’ and it contains cholesterol, wax, and wax esters as its main constituents. Its production can increase as a result of many factors :

Excess Androgens (Hormonal)

Use of certain cosmetics and topical medications (irritants,oily preparations)

Certain drugs and medications such as corticosteroids

Excess sebum production, in turn tends to irritate the epithelial lining of the duct causing a cellular hyperproliferation, and eventually increased proliferation of a commensal bacterium : Propionibacterium acnes, which is present normally in small quantities as part of the pilosebaceous unit.

All these factors tend to cause the blockade of the duct, the outflow pipe of our oil glands,causing it to swell up and thus forming a comedone.

In the initial stages, there is a partial blockade, such that air and therefore oxygen can reach the collected sebum, cellular debris and oxidise the surface of this collection causing it to appear ‘black’. This is what is commonly knownas a blackhead.

And when this blockade becomes complete, and the duct is completely obliterated, oxygen can no longer reach this collected material causing the formation of what is known as a whitehead or a closed comedone.

Hence, even though a blackhead appears 'black' on the surface, pressing it and discharging it's material leads to expression of primarily sebum which is yellowish to white and mixed with this cellular debris of the cells lining the duct and few bacteria, all in all producing an oily whitish yellow dirty looking material.

And here is a guide about how to remove blackhead.