OK, I've got SOME of the Bloodling/Vampire fluff written. More will hopefully be up tonight.
Vampires have been nearly extinct for nearly a thousand years now. Their Great Houses now nothing more than old tales and ruined castles. The event that caused this tragedy (depending on your point of view) is known simply as The Vampire Rebellions to common folk, but to the Bloodlings, it has another name: The Great Awakening. It started when a thrall of House Sliscus somehow broke free of his master's control, and killed the vampire whilst he slept. In doing so, all thralls under that vampire regained their free will, and together lead an uprising that saw the complete and total annihilation of House Sliscus in a single day. These newly freed thralls, now calling themselves Bloodlings, went on to attack other Great Houses, freeing more Bloodlings and gaining in strength. After a hundred years of fighting, all but two Great Houses had been destroyed, though they did not survive unharmed. In fact, so terrible were the loses suffered by these houses that it is unsure if either managed to survive much longer after the rebellion ended.
Now, as for what a Bloodling actually is. A Bloodling during the time of The Great Awakening was a vampiric thrall freed from the control of it's master. The modern Bloodlings are descendants of these former thralls. Now you might be asking "But what was a thrall?" A thrall was a human (or member of another near-human race) that was bitten by a vampire, but not turned. This caused the human to attain some minor abilities similar to a vampires, but at the cost of the thrall's free will. The two most important changes caused by thralldom were the growth of vampiric fangs, and the growth of an organ located opposite the heart, known as the Blood Hall Heart. The fangs allowed a thrall to collect blood just like a regular vampire. However, thralls did not require the blood of others to live. Instead of providing nourishment, the blood would be stored in the Blood Hall Heart, which could then pump the absorbed blood throughout the body in case of blood loss. This allowed vampires to gather blood during the day via thralls, and in other situations where a vampire would have trouble (or if the vampire was feeling lazy, as had become a common occurrence in the few decades leading up to the Awakening).
EDIT 1
Ok, now for details on Bloodling physiology. First off, they inherited many physical traits from their Vampire masters. This includes fair skin and a youthful appearance. However, unlike vampires, they are not immortal, though they do live on average to 150 years old. Instead, they age more slowly than humans, so a 50 y/o Bloodling may appear 30, or sometimes younger in certain cases. They do not possess any of a vampire's weaknesses, nor any of their strengths. They cannot transform, hypnotize with a gaze, etc. They're closer to Human 2.0, rather than Beta Vampire. As mentioned before, they do possess vampiric fangs, for sucking blood. They are slightly more dextrous than the average human, but no stronger, smarter, etc.
Now, the most important part of a Bloodling's physiology, indeed, the very thing that marks them as Bloodlings: The Blood Hall Heart. This organ is situated opposite the heart, and is similar to both heart and stomach in form and function. If needed, it can pump stored blood into the body, either preventing passing out from blood loss, or increasing strength for a short amount of time. Should the heart be damaged, the BHH can act as a backup until the heart is healed. Now, the most intriguing, and quite possibly most important, feature of the BHH is it's ability to transform blood stored within to match that of the Bloodling, down to the genetic level. It can even change an animal's blood into that of the Bloodling. How it does this exactly is still a mystery, frequently researched by Bloodling clerics and doctors alike.
And then there's Bloodling procreation. Early on, Bloodlings learned that they could no longer produce children with members of their original race. However, they could produce children with other Bloodlings, even those with a different origin race. The resulting child would appear human, or if both parents were different enough from human in appearance, the child would appear as a more human version of one of the parents.
EDIT 2
Ok, let's talk about society. Like the the vampires, the Bloodlings are divided into large groups/families, known as Fellowships (named after their church, the Fellowship of Blood). Each Fellowship recieved it's name from it's founder, usually an interest or favorite motif of theirs. The three most prominent Fellowships are the Fellowship of the Grave (deeply religious, the majority of Bloodling clergy belong to this Fellowship), the Fellowship of the Rose (heavily into art, beauty, aesthetics, etc. Also masters of diplomacy and intrigue), and the Fellowship of Steel (greatest military among the Fellowships. Also known for it's scientific advances. Claims to have been founded by the first Awakened Bloodling, Dimiri). The Fellowships, while rivals, share a sense of unity, and so far have never gone to war with each other, save for the Fellowship of the Dragon. The Fellowship of the Dragon is host to radicals and renegades, who believe that if they can return vampires to power, they will receive the gift of vampirism. They seek the destruction of the other Fellowships and the restoration of the two remaining Great Houses (House Sothos and House Praxis).
EDIT 3
Deities
Dimiri, The First
-Dimiri was the first thrall to break free from his vampire masters. It was he who started the rebellion that nearly wiped out the vampires. However, it is said that he died of old age, without blood spilt, and thus was trapped in Limbo. However, so great were his accomplishments that the powers of Fate elevated him to godhood. He now sits at the Gates of Death. Those Bloodlings who offer him their blood upon death will be judged, and sent to either Ascalon (heaven) or Moritus (hell). Those who do not offer their blood are turned away, forever cursed to reside in Limbo.
Szarek, Binder of Souls
-Szarek was the ruling vampire lord of House Sliscus at the time of the rebellion. He was personally killed by the hand of Dimiri. Upon Dimiri's ascension to godhood, he elevated Szarek as well, but binding him inside the blighted realm of Moritus, there to watch over the souls of damned Bloodlings.
Moroii, The Caretaker
-Moroii was Dimiri's wife, and in his last years, his caretaker. Though she died several years after Dimiri, he elevated her to godhood as well, giving her rule over the blissful Ascalon.
Afterlife
-Ascalon: The Bloodling's version of heaven. It is a blissful place, full of everything one would ever want.
-Moritus: A tortuous hellscape, consisting of volcanoes, blasted deserts, flesh-rending sandstorms, and near constant meteor impacts. Reserved for Szarek, vampires, and those Bloodlings deserving of such a fate.
-Limbo: A spirit realm co-exisiting with the mortal realm. Those trapped here can not interact with the physical world in any way, nor may they stray too far from their body.
Beliefs
Value of Blood: While Bloodlings don't require the blood of sentient creatures to survive (the blood they drink is usually animal blood), like vampires did, they still hold blood as their most sacred gift. When a Bloodling slays a foe, they drink some of the fallen's blood, believing they will take on their strength. At the same time, it is a great tragedy for a Bloodling to die without spilling blood. It is such that a Bloodling dying of old age on his death bed will have a cleric slit his throat so that he may spill his blood in death. The meaning behind this is that the spilt blood is an offering to Dimiri. Should they die without spilt blood, Dimiri will refuse to judge their soul, trapping them in Limbo for eternity.
Worship: While the Bloodlings worship their own three deities almost exclusively, they acknowledge the deities of other races, though they do not believe they have any sway over the soul of a Bloodling.