Since the setting is named for it, I thought that I'd discuss a little bit about what the Black Blood of the Earth is, in the setting. It is the necessary component of all magic. It is the way that internal states are transformed into external realities. It is the lifeblood of the land, flowing through the underworld like a vital fluid.
OK, OK, so what does it do? In order to cast any magic, a magic-user (or illusionist, or cleric, or even druid) must imbibe a small amount when preparing the spell. About 1 fluid ounce per spell level is sufficient. This is not entirely cheap, however, as each fluid ounce costs (currently) 3 gold crowns, 18 silver nobles, and 8 copper wheels. This price fluctuates over time.
My host in Belwick had a houseguest who harangued us all about the abuse (as he termed it) of the Black Blood. He claimed that the increasingly intense weather being experienced in the world is one result of the depletion of this vital planetary resource. No one, though, seems willing to give up magic for alarmist and unproven theories of that sort. This guest claimed that there are ways to continue to practice some magic, but that it can't be as ever-present as it is now, and that society will need to undergo drastic changes if it is going to continue without collapsing into chaos. Since he wasn't forthcoming as to what these "other ways" were, he was gradually ignored by the other guests. Most likely, though, he was talking about the Sun Elixir, which is a very expensive alternative to the Black Blood. It currently costs upward of 80 platinum imperials per fluid ounce, largely due to its slow method of manufacture (and the requirement for a long period of sunlight, which is just not practical in some areas). As a result, few people bother with it.
In addition to being used to cast magic spells, the Black Blood is used to recharge magic items. Each fluid ounce will help to restore one charge to most magic items that have charges. It is also necessary to maintain long-term spells - such spells as Continual Light require a number of fluid ounces equal to the level of the spell per month (the first month is covered in the amount required to cast the spell) in order to keep functioning.