As the title of the game suggests, much of the world in Bloodborne revolves around the importance of sinister celestial blood and its origins. Although, it is not a divine gift as many NPCs in the game would have players believe. The blood vials one consumes to heal are not an Estus Flask allegory, but something else entirely. The sanguine essence used to slake the desperate thirst of hunters indeed comes from the Healing Church, though there are no benevolent deities involved.
The Old Blood coming from this sanctuary is that of beings who are Kin to the cosmos; children of the Great Ones. Mighty as they are with blood that can heal, these powerful beings come from beyond and have no interest in helping humans. Their presence is never explained, nor are their motives, but humans in Yarnham have still taken to using their proximity for their own gain. At least, so they think.
10 Byrgenwerth Origins
The College of Byrgenwerth is arguably the most important location in the game in regards to the lore and origin of the events leading to the beginning of the game and the troubles the player witnesses when traversing this land plagued by those tainted by The Old Blood. For it is here that scholars from long ago studied the secrets of the many, many labyrinths where crypts and tombs corrupted by cosmic energy dwell.
It was in these sepulchers that the curious academics found The Old Blood. They experimented and investigated this odd liquid until they found properties in it that could be exploited. From these discoveries, some scholars then formed the Healing Church and claimed the use and proximity to this cold blood concoction to be akin to transcendence.
9 Milked For Blood
Ebrietas, Daughter of the Cosmos is a boss that looks like something straight out of a cosmic horror tale that can be found in chalice dungeons, though the most significant individual of this kind is located in the Upper Cathedral Ward. They are not a unique specimen and have multiple siblings, making their lore all the more ominous.
The one found in the ruins within the church grounds is the producer for much of Yarnham's blood supply, which she is milked for not unlike a very strange-looking cow. However, she is not bled or brutalized in the harvest, for her own blood (like that of all other true Kin) is yellow. She produces the red liquid, which looks very much like human blood, by spewing it from her head.
8 Mother And Child
Horrific coupling in all manners is implied throughout the course of the game and very much pertains to the Healing Church's experiments as well as the unknown motives of the Great Ones from beyond. Ebrietas, Daughter of the Cosmos, is not a being that organically came to be. She bears a close physical similarity to the Celestial Children found throughout Bloodborne, all of which are the infant forms of Kin creatures.
These Children were created from human women who have been impregnated by celestial fathers, though the means of which are not explained. However, this can be proven by following Arianna's questline. If she is found and sent to Oedon Chapel, she will eventually begin to show signs of severe abdominal pain before being found at the end of a trail of blood, not too far from her usual spot in Oedon Chapel, next to her newborn baby: one of the Celestial Children.
7 Raised For A Single Purpose
From these abhorrent births, it can be theorized that the Healing Church, ever in search of knowledge and power, is responsible for this breeding of human women with Eldritch fathers to create Celestial Children, who are non-hostile and simply amble about whenever found (particularly in areas near church structures, where they congregate and seemingly pray).
This lets players take a good look at them to see that their slug-like forms house two skeletal wings, very much like that of Ebrietas. Since this recurring boss is seen multiple times and is referenced as a daughter of the cosmos, it seems like Ebrietas are Celestial Children who have reached maturity and are ready to be used.
6 Yellow & Red
The Ebrietas found in the main game, in the Upper Cathedral Ward, is also non-hostile when first encountered. If players look around the area, they can see her gazing at the crushed form of another Kin, one very similar to a different boss called Rom, the Vacuous Spider. The reason that Erietas is here, on the church's property, is because she was likely drawn here as a Celestial Child, similar to her tiny siblings, and then imprisoned for the Healing Church to exploit by milking her.
Since she is of both cosmic and human origin, she has both types of blood. The red variety of which holds special properties valuable to humans as a healing concoction. And her yellow blood, which emerges when she is cut, offering little use to humans but betrays her extraterrestrial parentage even more than her bizarre form.
5 Vermin
These disgusting consumable items are parasites that reside within humans, beasts, and Kin, though do not affect the latter in the same way as the formers. This may be an indication of the trend happening in Yarnham. It is implied that the use of blood healing has been going on for quite some time, and in fact, the usage has been increasing.
Much like what happens when fluids are exchanged heavily among organisms in the real world, things were transmitted from individual to individual in Bloodborne. These centipede-like pests don't have a clear origin and simply appear where there has been a lot of tainted blood. This can be taken to imply that these vermin are simply parasites who live in blood and reproduce inside living hosts. If their host's blood gets into another, then the vermin spread. It is not explained if these pests came from human, beast, or Kin, but throughout the game, players can find hideous examples where these parasites take over their hosts, like with the boss the Bloodletting Beast, which has a giant version of a vermin emerging from their head and seemingly controlling the body.
4 Blood Echoes
As a name, "Blood Echoes" are odd, though not many have delved into the meaning. They are a red liquid that is full of lingering memories and ghostly voices. At first, this seems like they are simply the remains of the player who is collecting their lost echoes after death, though this is not the case.
There is a being, more powerful and mysterious than all other Great Ones, who is linked very closely to both blood and the sound of disembodied voices. Oedon, an anagram for "Odeon" which is a structure used for musical performances, is a bodiless ancient being that has had great influence over the Healing Church's quest for The Old Blood and the history of Yarnham.
3 Oedon
The only Great One in the game not to have a form, or even mentioned to have a form, is the ancient and mighty Oedon, who even had structures named after them. They were a being who ascended to a higher existence or a different plane of reality, even higher than the other Great Ones, who are still mostly corporeal.
Oedon is mentioned to persist in this world as a presence more akin to a god than a living organism, and they are very closely connected with blood, being mentioned by in-game sources to be seeking more of the red sanguine syrup. Proof can be found on the Caryll Rune aptly named "Oedon Writhe", for it states that the thick slime upon this stone may not be human and is the essence of the Formless Oedon who seeks blood.
2 Yharnam, Pthumerian Queen
Initially appearing as a creepy apparition following the boss fight with Rom, the Vacuous Spider and again in the Nightmare of Mensis, this ancient queen heralded the beginning of the end for the city that was named after her. Yarnham, Pthumerian Queen, was from the ancient civilization that once made pacts with the Great Ones.
This pact took the form of Yarnham linking with Formless Oedon and becoming pregnant with their child. It is likely her blood and existence that the old scholars of Byrgenwerth discovered while delving into the Chalice labyrinths that led to the cultivation and farming of The Old Blood. Since the land shares a name with her, it seems very likely that the Byrgenwerth College, then the Healing Church, was not only aware of her but revered her.
1 Mergo
Yarnham's child, fathered by Oedon, is the Great One Mergo. This infant was not long in this world, however, as they do not appear in-game in actual form. Only as a reference residing within a silent carriage guarded by their inhuman wet nurse. Mergo is the creator of the dream in which hunters find themselves, though the state in which they dream is a twisted realm borne from blood inherited by their human Pthumerian mother and cosmic Great One father.
The Pthumerian queen, Mergo's true mother, can be found just outside the boss room where the stillborn baby and their guardian can are located. After defeating Mergo's Wet Nurse, the last mandatory boss in the game, and the mysterious monstrosity acting like the dead child's surrogate mother, Yarnham will bow to the player, as if thanking them. It has never been confirmed why she would be grateful for this act, however, from all the other information gleaned about blood so far, players can assume that she is grateful that the horrid reality caused by Mergo's undying nightmare seems to be coming to an end.