The nature of magic.

This is best described via a somewhat extended metaphor, so bear with me on this.

Think of magical power as a well, filled with a thick  liquid that settles and gets denser and heavier as you reach further down into its depths, thinner and lighter at the surface. The denser the liquid, the more powerful the magic.  Thus, the deeper you reach into the well, the denser the material you draw up, the more difficult to lift and hold it, and consequently the more powerful the magical effects you can create. While the heavier and denser magic is more difficult to handle in terms of the strength and willpower required to use it, using it is also faster and in some ways simpler than trying to create similar effects using the lighter power towards the top. The thinner stuff at the top must be hoarded slowly over time, drawn in and pulled together with rituals and tools, with great skill and patience, to create small effects.  The stuff at the bottom is the very essence of power: draw it up, frame a purpose in your mind, hurl it away, and you can level mountains.

But, the well has been poisoned.  Magic was inextricably tied up and tangled with the gods and their power, as they created it as a tool for their vassals to use. Thus the corrupt blood of the gods, when it was spilled, infected all magical power.

The corrupt blood, to continue the metaphor, is the deepest and densest power available. Thus, the bottom of the well - the strongest magic - is pure blood and corruption, and the corruption gets lighter and more sparse as one rises up to into the lighter and clearer power. However that corruption is not limited strictly to the bottom, some will always diffuse upwards.  Any use of magic has at least a small chance of touching on that corruption.

There are ways of doing magic that are very safe, if never perfectly safe.  Alchemy and enchantment, by working slowly and carefully, accumulate small amounts of power slowly, skimming the very top of the well over a long period of time, and minimizing the chance of corruption. Alchemists will say that their works are completely safe, but of course there is always some risk, however small. With great skill, and great patience, wonderful works may be created this way, with minimum risk, albeit slowly.

There are  pools and rains of Blood that are physical manifestations of the metaphysical bottom of that well of power.  That power is so great and so dense that it has an actual physical presence, manifesting in the physical world even when not being actively used. 

It is technically possible to be a classical D&D style sorcerer, without touching the blood. But it requires tremendous skill and not a little luck.  One must be deft in drawing power from the well, knowing how deep one may reach and avoiding the wisps of corruption that reach upwards.  Some succeed and attain power without risk.  However most such practitioners eventually pay for their daring.  It is difficult to sense the Blood if you have not already been corrupted, and thus it is very difficult to avoid those thin wisps of Blood that rise up through the well for very long.

The metaphor of the well is so widespread that those who fall to the blood have long been said to have "drawn too deep". The phrase has been generalized over time, and now anyone who does something foolish and dangerous or self-destructive is said to be "drawing too deeply", "diving" or "swimming in it".

I want one thing to be completely clear:  once one touches the Blood, even the tiniest amount for the briefest time, corruption is inevitable.  That person is, in one sense or another, doomed, without any recourse and without any hope of redemption.  The corruption can, with skill and willpower, be held back and repressed for a time, the process slowed, but it cannot be entirely stopped. The most one can do is hope to die by other means before the corruption kills them, as any other death they may have will surely be a kindness compared to the end that the Blood will give them.  The effects of the Blood cannot be cured or undone.

Of course, the mage who finds that she has touched the Blood, be it accidentally or deliberately, with their hands or even just with their minds, does in fact have  a choice as to how to face the consequences.  She can fight the corruption, or she can embrace it.

The Mage who chooses to fight the inevitable will die, disfigured and mad, and in horrible pain. All she can do is delay the inevitable.  She can try and ignore it completely, or try to fight the corruption, and either way she will descend - quickly or slowly, but inevitably - into madness and eventual death.  Ignore it or fight it, the result will be the same in the end, though with willpower and self control, the descent may be slowed. 

On the other hand, she may bow to the inevitable, embrace the corruption, use it consciously and deliberately, and she will transform eventually into one of the intelligent undead, the "Blessed".