Dirty sebacious matter clogging a pore to the point it is visible to others. Most often found on noses, cheeks and chins, though they can develop anywhere. They are often satisfying to squeeze. They are also a sign the person does not take very good care of him- or herself.
My ex-wife, who turned out to be a pig, let her whole nose get full of blackheads. Everyone thought they were freckles.