Refers to the descendants of humans who consorted with something “inhuman” in the distant past. Tohno Akiha and Kishima Kouma fall under this classification.
Though their blood is growing thinner with each successive generation, on rare occasions a child is born who has strongly inherited that “inhuman” characteristic.
As for Tohno Akiha, her blood is primarily human, but what inhuman blood she does have is exceptionally high class. Because of this, she could be considered the answer to the demon hybrids’ evolution, capable of awakening as a demon hybrid while still remaining a human being.
Kouma, on the other hand, is an archetypical demon hybrid whose oni blood runs too thick, making life as a human completely impossible.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary

Those that have inherited the blood of ancient purebloods, like the Tohno or Saiki.
Pureblooded species means non-humans (oni-kind), or alternatively those that have strayed from being human and turned into onis.
As they have inherited the role of nature’s sense of touch (output devices), they have abilities that are different from those of psychic ones.
Psychic abilities are ultimately something which the human performs on their own, while the abilities gained through mixed blood is that of natural interference/natural contact—not something manifested by just one’s own personal abilities (one’s brain).
In addition, psychic abilities (generally) abide by physical laws, while the abilities of mixbloods often do not. Fantastic stories, folklore, tales of old—in order words, unnatural phenomena which cannot occur in reality.
Psychic abilities and mixbloods have a relationship similar to that of magecraft and magic.

Katatuki Kouhon: World & Tsukihime Character Setting