Plane of Blood

As a lesser plane, it was possible for a single higher entity to assert almost full control over it: doing so for one of the fundamental planes would require a level of power even gods could not reach. Having dominion over the plane would then allow the entity to shape its resources and grant its powers selectively. In different eras, the plane was ruled by different beings with different approaches: at one point, the Plane of Blood was used to create beasts summoned by the deity's followers. During another, the entirety of the plane was used as an extradimensional 'body' to manifest a sole entity's avatar.
Since the original age of chaos, a semblance of an ecosystem has evolved within the plane- based upon stray creations left unharmed.

Why isn't *everything* alive Blood-Aligned?

The Plane of Blood is a lesser plane- while concepts associated with it can be used to invoke it it, they do not inherently do so. Heat always naturally increases alignment with Fire, shadows with Darkness, and so on. Shedding blood does not inherently call upon Blood. The Eideli's bloody field ritual only had meaning because they, as a culture, worshipped the current ruler of the Plane of Blood.
And on why it's a lesser- Blood is zero-sum at best. It's like the inverse of Fire, which gives more than it takes. Relying entirely on Blood causes regression every generation. There were eons where if Blood is ever the majority 'shareholder' in a civilization's magic, it generally burns itself out over a generation or two. Blood is the magic of usurpation, of overreaching, and unsustainability. This isn't to say that Blood doesn't allow 'breaking' of conservation of energy, just that unlike the others, someone had to pay for the difference at some point. The fact that such a vast sea of former sacrifices now exists means that cultures which manage true planar invocation to directly draw from the plane are unlikely to run into this issue, but those that essentially call upon it as a library to perform operations/functions hit this zero-sum wall hard. In the 'modern' (post-Chaos) era, this downside is largely irrelevant due to the vastness of Blood's reservoir, though it's by no means indefinitely sustainable.

Concept

One of the underlying concepts of Blood is the lack of 'true' spirits. Blood-aligned creatures tend to be masses of biological matter manipulated through magical means, but far more entrenched in physical existence than one would expect from a spirit. They're sort of a bridging point between mundane life and spiritual.
Human's souls and consequent flubbing make them slightly more magical simply by existing and exerting their will upon the world subconsciously. Wastelanders, or ghoubloods, take this a bit further due to their heritage, being more adept at creating 'impossible' physical effects with Xiphaem . Ghouls are of course xiphaem-driven, but often use this tangentially by making physically plausible constructs of flesh, bone, nerves etc. with which they can manipulate by ordinary physical means. Slimes are entirely directly xiphaem-driven.

The blood itself

Blood within the plane doesn't deteriorate, though it can be forced into certain stages through application of magic: this is why there exist islands of clotted and dried blood dotting the land while the sea of blood never shows any signs of thickening.
Obviously, all the blood is AB+.