Boons

Boons are bonuses that can be chosen on floor 2 of The Dungeon. The first three provide a one-time bonus, while the fourth, secret boon confers a permanent bonus and handicap. They are not unlocked by default, but must be acquired by beating various bosses.

Strategy
From a purely gold-based point of view and of the blood boons, Blood of Illifar is almost always the best choice at higher difficulties. An experience potion costs 3 gold from the Witch and a character in act 1 costs 3 gold from the campfire, so the boon is worth 12 gold. Of items that appear in stores, and the average price is 6.7g, so the Boon of Nazzatoth is worth that.

In general outcomes, the 8 gold is almost always spent purchasing an extra hero at the first campfire, meaning the real translation of value is more-or-less a level 1 character of your choice and 0-5 gold (depending on reshuffles at the campfire) whereas the level 2 character is random. The advantages of having a level 2 character for act 1 are usually better than the bad luck of randomness.

If your starting relic is Counterfeit Coins, Royal Insignia, or The Merchant's Secret, usually the increased relative value of gold makes the Blood of Abaddon equal to or greater than the Blood of Illifar.

The Soul of the Keeper is usually only feasible with Troll Blood. For instance, in The Dungeon, you fight 6 battles and therefore take 6 damage from the Soul. Assuming you visit each campfire and buy a healing potion at each, you can restore 3 health. The end boss of the act provides an additional potion meaning the net negatives in act 1 are 3 gold and 2 health for your hero. By act 2 you could have purchased a Soul Stealer to partially mimic the Troll Blood, and in act 3 you could have The Priestess to restore hero health, so the negatives are not as prevalent there.