Perhaps no-one will be surprised, but I have power concerns on Necropotence and Dark Ritual. Dark Ritual doesn't seem insane, really, but I think it may be stronger than the other colors 'per-boon bonus' boons. Just keep an eye on it.

Necropotence gives you unlimited spells per day, by virtue of Cure Light Wounds costing 3HP to cast and restoring 1d8+5 HP per casting. So, you easily (boon and a 1st-level white spell) have infinite health, which this boon immediately converts to infinite spells. Or you could use a wand or something, point being the drawback is very easily overcome. Heck, even without healing, it only costs 18 HP to get a free 6th-level spell, and at level 13 (when you first get this) you will have 62 HP with average rolls and 10 Con, so 3 free max-level spells at least.

The flavor is spot-on however, trading your life force for magical power. So, there are two ideas i have for balancing this.
1) Increase the damage to 5, 8, or 10 HP/spell level. 5 isn't a big jump, and is still easily healed, but 8 isn't shrugged off so easily, and 10 makes it dangerous to use on high-level spells.
2) Have limited uses per day, perhaps 1/2 class level.

I'd suggest either increasing the HP cost to 5 plus some uses/day limit(a significant number of 'free' spells, but not infinite), or increasing the cost to 8-10 with no daily limit (high enough cost that healing yourself after use is non-trivial).