A blackhead is also called an open comedo (single for comedone), and it is a clogged pore in the skin that is open to the air. Keratin (skin protein) and sebum (oil) combine to block the pore. They are often found on the face and trunk. but they can be found anywhere on the body.
Blackheads are not clogged with dirt, but it is the exposure to air that causes oxidation turning the internal contents black. They can be extracted using a comedone extractor.
I usually use an 11 blade (a blade that comes to a sharp point) and a Shaumberg type comedone extractor.