It has been established that the Man in Black is the Smoke Monster and has now taken the form of Locke.
Illana says that since Jacob is now dead that the Smoke Monster cannot taken any other form other than that of Locke. http://lostpedia.wikia.com/wiki/The_Substitute
Back in episode http://lostpedia.wikia.com/wiki/The_Man_Behind_the_Curtain Ben follows his mother into the jungle and then has a meeting with Richard who asks whether his mum died on the island?
We now know that the Man in Black can take the form of dead people whose corpses are on the island
But what about people who died elsewhere?
Can Jacob perform that feat but the Man in Black cannot? Was the vision of Emily Linus a manifestation of Jacob?