How do we necessarily know that Flocke is the man in black? If you watch the season 5 finale the MIB showed up at Jacob's chill spot, Like he knew he was there, like he knows him. In the very same episode Flocke had to get Richard to show him where Jacob was...
he showed up at jacobs spot, because richard led him there. he didnt know that Locke was dead. He thought he was leading the real Locke to the statue.