If my duskblade has temporary hit points and causes himself 5 hit points of damage using the Blade of Blood spell, would the damage come off the temporary hit points, or his regular hit points?
Essentially, temporary hit points are typically lost first, before a creature's normal hit points are lost (Player's Handbook 146, Rules Compendium 72). It usually doesn't matter how the creature loses the hit points as long as the hit points are lost due to hit point damage. (For example, a creature that suffers from Constitution ability score damage might seem to lose hit points when, in fact, the creature's maximum hit points have been reduced, so its temporary hit points are unaffected.)
The 1st-level duskblade spell blade of blood [necro] (Player's Handbook II 103) specifically allows the caster to deal himself hit point damage; this damage is typically subtracted first from the caster's temporary hit points, if any.