Board Thread:General Discussion/@comment-25926288-20151116014923/@comment-6175354-20151117191739

Farerb wrote: So Hook wouldn't have been called a Dark One. But that is not what is said and shown on the episode. Hook is a dark one = there's darkness inside him. Hook is unfortunately a Dark One, because he was reborn from the Dark One's Vault.

Merlin (like all people) did have darkness within, he just chose not to act upon it. Hence, why he did not corrupt his magic. From what it looked like, Emma transferred Merlin's magic and immortality into Hook. In doing so, she corrupted the magic.