Board Thread:Spoilers!/@comment-6175354-20140804052626/@comment-24674051-20140814161946

Another thought on the blackmail is that it is Emma that blackmails Gold. Since she is the sheriff, she could decide to go back and review the tapes of Zelena again, and when Gold is not there to mess with the tapes, she discovers the true story.

As for why, best guess is that it would relate to her guilt on bringing Marion forward and ruining Regina's happiness, and her trying to find a way to fix it. (Or maybe SHE is trying to persuade Gold to reattach Killian's hand).