Blood:
The energy pool your talking about could be the casters own blood.
To regain that energy, sleeping and general restfulness could increase the amount of blood in the body, so that the caster can cast more effectively. But this is boring.
Combat:
While the casters own blood is most potent for his or her spells, the blood of others works too. They need physical access to it, its your call whether they need to drink it, stand in it, whatever. The caster can use lethal quantities of other peoples blood to cast too. The more you kill, the more you cast.
Casters:
Their blood is better than muggle blood. A combat caster would specifically target enemy casters, with the intent of capturing and harvesting their blood, which you go with your Universe, is probably also an integral part of the energy-recharge potions you mentioned.
Naturals vs Meditators:
This could either be hand-waved, or if its a hard requirement, then meditators have a mind-over-body way of having their body produce more blood than normal, similar the the Buddhist monks who can raise their body temperature and change their heart rates at will. It is not inconceivable that this skill or ability would appear in a world where blood was the determining factor in who lived and who died.
Politic:
The Crimson King and Vampire clans and all sorts of nasty political entities would form around the need to acquire large volumes of blood to win wars. 227
Others would band together to protect innocent people and rally the masses to their cause. For this reason, I don't really see 50% and being sustainable.
50%, but for how long?
This blood-caster trait would get pretty noticed. Maybe it would go like Dragon Age, where using other peoples blood becomes heresy and gets you killed. More likely, the muggles would rise up and hunt casters, vilifying them and calling them things like "Vampire". Or perhaps we would be something like the goofy last season of True Blood, where everyone would try and get a caster in the family, making the trait so Universal that everyone has it. Either way, I can't really see 50% holding for long. A couple hundred years, tops. That gives all the evil casters time to win large wars or all the muggles time to hunt casters to extinction. 164