I'd argue affliction locks are the closest to necromancers in the game.
Good luck. They're perhaps the most powerful beings in existence barring the Titans and Old Gods. There's a reason they're not Warlock pets. Like others noted, DHs are not a pet class.Enslave Eredar: Grants extra Threat and Defense rating.
The demo spec could, with tweaks, accommodate the DH concept. A "hero" class devoted to it is therefore a waste of time. Warlocks have almost all the DH's powers. Re Blood Mages, they're blurry, both in the Mage and Warlock camps.