How could they blackmail him? What could they possibly threaten him with?
Exposure of his real birth. Think Mossad might know? The Chicoms?
I pretty much expect that people within the CIA, FBI, etc. have looked over that online copy. And if they came to the same conclusions as the guy who says it was a fraud based on a manipulated scan of other COLBs, then they have two options. Go public with this information, or blackmail Obama for their own agenda.
He could become a puppet to certain people in government who could push to expose him as a fraud and that he has no legal authority to make any decisions a POTUS.