Most blackmail as the previous 2 responders have said would fall apart once the 'criminal' act occurs. For it to truly work, it would be a rare set of circumstances that takes a lot of prep work & reconnaissance of the target in order to see if there is truly something that would be blackmail-worthy. Often times, it's not...but in terms of how it plays out in a story/fantasy...well...that's completely different.
haha