Grand Master Helminter:
Regardless of your occupation, you must reach at least level 20 before you can obtain the right to transfer to another class. Likewise, in order to become a Dark Wizard, you must be at least level 20, and you need proof that you have passed the test to become a Dark Wizard.
Your level is sufficient, but it seems that you have not yet passed the test to become a Dark Wizard. In order to become a Dark Wizard, you must go to Witch Varika at The Altar of Rites. You must pass the test of a Dark Wizard that she gives and bring back the proof Jewel of Darkness in order to become a Dark Wizard.