Enlarged pores are usually due to genetically predetermined oil gland size and density.
These are different than true blackheads, although many people call any enlarged pore on their nose a blackhead.
Sebum, which is the oil that is secreted out of the pore, turns black when it hits the air (oxidizes). This is not dirt, but a chemical reaction. A blackhead is an open pimple in which the oil and other things that are secreted by the glands get stuck and creates a plug in the pore. One often hears that you cannot change the size of a pore, you can only make it look smaller. In many ways this is true. If you can keep them empty and though, they tend to look smaller.