The Blood Shrine offers two rites, and both are paid for out of your own body rather than your purse. That makes it the one place in the game where the cost is measured in the resource you were about to spend anyway, which is exactly what makes the timing worth thinking about.
An offering costs 40% of your maximum health and returns a 25% faster energy regeneration for 60 minutes, on a 6-hour cooldown.
Read that trade carefully, because it is not a power buff: it is a rate buff. The offering does not give you energy, it makes energy arrive faster for an hour. Its value therefore depends entirely on whether you are going to be spending energy during that hour. Making an offering and then logging off wastes the entire cost, and the cooldown means you cannot simply try again.
The health cost is also a real risk, not just a number. Going into a fight or a risky action at 60% health after an offering is how players lose runs they would otherwise have survived. Offer at the START of an active session, when you are safe and about to spend heavily, and heal before you fight.
Shadow Mending converts mana into health at 1 mana per point of health, up to 50% of your maximum health per rite, on a 30-minute cooldown.
This is the cheaper of the two rites in practice, because mana is the resource most characters have least use for moment to moment. If you are not running magic-heavy actions, mana sitting at full is idle capacity, and Shadow Mending is the way to turn it into something.
The per-rite ceiling and the cooldown together mean it cannot replace potions in an emergency. Treat it as scheduled maintenance between fights rather than as a heal you can rely on mid-crisis.
The offering sits on a 6-hour cooldown and Shadow Mending on 30 minutes. Those two numbers are what stop the Shrine becoming a treadmill, and they are very different in character.
The mending cooldown is short enough to be part of a routine: you can expect to use it several times across an evening. The offering cooldown is long enough that you get at most a few per day, which means each one is a decision about which hour of your day is worth accelerating. If you make an offering carelessly you have not lost much health, but you have lost the hour you would have wanted it for later.