As the flow of oil clogs up, it mixes with accumulated dead skin cells that haven't exfoliated as efficiently as they should, narrowing the 'exit' of the pores so that the sebum cannot be secreted naturally.
Try Toothpaste Solution Blackheads basically happen when oil and dirt get caught in the pores and then solidify there.
Heat the mixture for 5-10 seconds in a microwave until they form a paste.