Blood Servants are pseudo vampires that have contact with first generation vampires (primogenitors). It is shown in volume 8 that by having contact with their master, the person can summon their master's beast vassal.
Blood servants are created by 3 ways:
Consuming the blood of the master vampire:
"The problem was if blood plasma from the vampire’s own body flowed from his fangs as they still pierced, and into the other person’s body. Those who received blood from vampires became “Blood Servants.”
“The possibility is quite strong that a direct descendant of the Fourth Primogenitor would be a vampire surpassing Duke Ardeal in power,” said Black. “Or we could drink the blood of a primogenitor and become Blood Vassals ourselves—” White pointed out. “…So, how ’bout you give me a shot?” said Red, standing right before Kojou and pointing to herself. Kojou was in shock at how overly blunt her statement was. A Blood Vassal was a pseudo-vampire; they could only be created through vampiric contract by first-generation vampires. They were said to possess combat capability sometimes surpassing that of pureblood vampires and lived with their masters for eternity. The girls’ objective seemed to be to become Kojou’s Blood Vassals and to gain combat power on par with the Fourth Primogenitor himself. Kojou actually felt relieved that their behavior to this point was coldly calculated and not based solely on sexual desire.
Taking a piece of the master's body and wearing it:
Though it was a common folktale, becoming a vampire from a vampire drinking your blood was not complete nonsense. A human being that had taken in a piece of a vampire’s body obtained a bodyjust as immortal as the vampire lord’s own. These were the pseudo-vampires known as Blood Servants. “…Wait, you made Himeragi into a pseudo-vampire?!” Kojou glared at the black cat in a rage. Inevitably, pseudo-vampires lived every day, month, and year of their eternal lives with their vampire masters. This was by no means a blessing. And yet, Yukari Endou had made Yukina Kojou’s vassal in spite of this, to the point of ignoring what Kojou and Yukina, the actual parties concerned, thought of the matter. “Well, properly speaking, Yukina is not a genuine Bride. That ring is merely acting as a ritual spell catalyst to create a spiritual pathway. If I had to put a label on it, she is akin to a fiancée. After all, were she to become a complete Blood Vassal, she would lose the ability to use spiritual energy.”
Repeated biting of the person:
"The problem was if blood plasma from the vampire’s own body flowed from his fangs as they still pierced, and into the other person’s body. Those who received blood from vampires became “Blood Servants.” That didn’t mean it was guaranteed to happen. That probably changed according to the phases of the moon, the condition of the human being’s body, and one’s ability to resist dark forces. However, if vampiric behavior was done over and over, eventually the other person would become one of the undead. That meant living together as partners for the rest of one’s eternal life."
They obtain a body that is immortal and have magical power the same as their master.