Just got the book yesterday. I've skimmed most of the fluff and read through most of the crunch, particularly the Black Magic amd the bad mojo is indeed sweet.
Black Magic works mechanicaly as a form of Tainted Sorcery. It's powered by the Casters Hatred and negative emotions, real Darkside of the Force stuff. In addition to the Manipulation Skills there are three other laws governing Black Magic, Agony, Hatred and Sympathy.
Agony lets the caster sacrifice HP to boost their chances to cast a given spell. Even better if the Wizard has been dealt a blow the round before he can Channel that damage into the spell as well.
Hatred is a very powerfull mechanic, a little too powerfull it seems. Hatred is factored into any of the Spells you cast that deal damage. If it's a loved one you are attacking theres a penalty to the damge. If it's the fellow who wiped out your family and friends his head explodes. Hatred allso gives you bonuses or penalties on Spell's skill roll.
Sympathy is the coolest of the three mechanics. It applies a bonus or penalty to the Manipulation roll depending on how well you know the target. If you something the target touched, owned, cherished or a "sample" of the victim you can get a bonus without knowing them. At the highest levels the Magnitude is increased.
Corruption is the balance to all this power. One cannot channel dark and fell energies without it effecting oneself. When the caster learns a new Black Magic spell, or casts a Black Magic spell there is a chance he becomes corrupted. If the Spell in question used Agony or Hatred there's a good chance corruption will occur. The corruption table is somewhat of a Smorgasbord of banefull mutations and mental abberations table. In many ways it seems more like the Dark Powers checks from Ravenloft and less like RQ mutations.
Another downside to Black Magic is that it is slow. The fastest spell in the book is a casting time of 5. However a Wizard can creat Foci that allow one to cast spells in a hurry.
The Black Magic rules are easily the coolest part of the Book for me. While I would have prefered more that four sample spells in the book it's easy enough to adapt RQ magic to it.