Great question! As far as I am aware, no one has asked that before!
The answer is NO - the Warlock does not count as a Dark creature. He is trained in the Dark school, but does not count as being a Dark creature. For a creature to be considered a "Dark" it has to be spell, not a Mage, and have the Dark school icon on its spell card.
To help make better sense of this, consider the Wizard. He's trained in the Arcane School, which summons all manner of weird aberrations and monsters, but he is not an Arcane creature himself.
I am going to post a clarification in the coming FAQ. Thanks for checking in!
