A comedone that has an opening to the surface of the skin (open comedone) is also called a blackhead. One that does not have an opening to the surface is a closed comedone, also known as a whitehead. The stuff that comes out of a blackhead is primarily keratin (shed dead skin cells) and sebum (oil).
Simply so, why do I have a lot of blackheads?
Blackheads form when a clog or plug develops in the opening of hair follicles in your skin. Each follicle contains one hair and a sebaceous gland that produces oil. This oil, called sebum, helps keep your skin soft.