Dreaming of blackmail means you are giving up power if you are the one being blackmailed. There is something that someone wants from you, and they have gone to great lengths to extract it from you. Do not fret, and do not submit to their demands is the only way to retain your strengths and sanity.
If you are the one doing the blackmailing in the dream, then it means you want to attain something easily without doing any work. Beware that it will end badly where your prize slips away.