A Simple Theory on Blood Magic
All mortal races of Thedas are made up of three elements, all equal in quantity:
Fade/magic/soul (Blue), Blood/will (Red) and Ka/life-force (Yellow) - (more on Ka here)

Alternate states and their meters:
Tranquil

Spirit

Old God Soul

Old God Soul in a body
Blood Magic
Things to note regarding the use of blood magic:
> The "Veil" is referred to in The World of Thedas as less of a physical barrier, and more an idea. Perhaps it should therefore be treated, then, as a level of resistance between two states. Where the Veil is said to be "thinner", the resistance is less. Moreover, less effort is needed to either draw from one's personal fade reservoir (magic) or "cross over" either way.
> Elven blood is said to be particularly good for blood rituals. It is perhaps possible, then, that their "will" meter capacity is slightly larger than other races, looking more like this:

The World of Thedas tells us that:
A mage draws upon a personal reservoir of mana to cast spells. As breath is to breathing, mana is to magic. When a mage's mana is depleted, the mage must rest to recover it, much as a laborer must rest after physical exertion [...] Various potions and artifacts, as well as learned techniques, can speed up the process of mana recovery.
Since there is a different section for "Blood Magic" I am assuming all of this applies to regular magic. So, when a mage casts a spell, they slowly draw from the "Fade" meter:

Although, as is the nature of the "Fade", this meter is malleable and can be replenished over time, or by giving your fade-shadow/soul some TLC. Since a mage's fade-shadow/soul unlike other spirits of the fade is connected to them, the "cap" of their meter is a reflection of the size of their reflected "sense of self". Therefore, a mage will always have a "max" level of mana which the meter eventually replenishes. We see that cap gradually grow when our character levels up and becomes stronger overall. (more on fade-shadow souls here)
Casting a Blood magic spell, predictably, draws on blood as a power source, so this begins to deplete instead - in exchange for a larger pool of potential Fade-draw and "choice".

It may also, as we have seen in previous games, give access to a range of abilities uniquely powered by blood or "will".
Although Lyrium will allow a mage to send his conscious mind into the Fade, blood would allow him to find the sleeping minds of others, view their dreams, and even influence or dominate their thought.
Using blood/will as a power source gives a mage the opportunity to not only travel the Fade, but use the existing link he has with fellow mortals to filter through all the formless and malleable content of the fade and hone in on another dreamer. This link is the ka, life-force.
So when blood/will is used, the mage not only extends the personal Fade reservoir pool, but also the ka capacity.

Now, the nature of the Fade as mentioned before is malleable, and largely a reflection of, and shaped by, choices. Solas tells us that Blood Magic is not dangerous by itself - only the choices we make through using Blood Magic. He also tells us that the more Blood Magic is used, the harder it becomes to use/enter the Fade.
When a blood magic spell is used, the "Fade meter" can potentially bounce back to it's original state over time, depending on how the blood magic is used. However, the ka is a more permanent substance, and once extending its capacity, remains extended.
One might interpret this as having another potential power source, but, unlike the others, that is not what this bar represents. Even though the ka meter is necessary for life, it is also a level of resistance from the "song" it is connected to. The natural, mortal size of the ka meter is arguably not particularly susceptible. But the greater the size of the ka meter becomes, the more susceptible the individual becomes to the "song".
So even though the "Fade meter" can bounce back after a blood magic spell, the ka meter does not.
And of course, blood replenishes as one would expects wounds to replenish. Except in the case of whole-body sacrifices... through making "violent" choices like this, it is possible for a mage to completely and permanently lose their "Fade meter" altogether:
Although, this illustrates a similar state as a tranquil... unless their fade-shadow has no way of disconnecting from them, and remains reluctantly attached to them, despite having no "Fade reservoir" to draw from. But it explains why evil magisters suddenly get all crazy about looking for Gods, by following the music....
Edit: I want to also suggest that a mage dying in this state is lacking any significant Fade "presence", and their entire self is therefore denied access to the Fade on death...
Also, that the void where the source of the ka and the "Forgotten Ones", originate from, can only be visited by beings lacking the "Fade meter" (i.e. tranquil or equivalent) without being affected horribly.
If applying this process to recent discussion regarding the Dwarves, it can be speculated that, if the dwarves were "witless, soulless" workers of "The Forgotten Ones" at one point, they would have also looked like above (in the tranquil sense). However, at some point their "souls" were crafted for them. Dwarves became connected to an artificial replacement for the Fade - "The Stone".

In theory, "The Stone" works for Dwarves in exactly the same way, except their network includes only dwarves, and does not connect them to the Fade (which unfortunately is the source of dreams and magic... which I assume was more difficult to replicate).
The Old Gods have the unique ability to draw on ka for power - but, since they essentially are ka (the "first children"), their natures are static, perfect and unchanging. Due to this it can be assumed that each Old God has a fixed purpose or duty to them. Their abilities relate to that duty. It makes sense, then, that the Foci Solas uses and gives to Corypheus contains the single ability to create rifts, using a limited source of ka within.
It has been argued recently whether the Elvhen Pantheon "Creators" and the "Forgotten Ones" were "Old Gods" themselves, or the "chosen" priest-types of the "Old Gods", imbued with their abilities. This theory can apply to both - as we can refer to the " Old God" ka as existing by itself, or in a mortal body.
(The Blight...I think might be an artificial attempt at overriding the ka, but still trying to figure out)