Blood
The Blood aspect, like some others, has not yet been shown to have a literal or clear-cut meaning. Common theories include affinity, affection and unity in general, such as the word is used in "blood brothers". Karkat, the Knight of Blood, had a reputation among his friends for sympathy, and he was the only one of the trolls in Homestuck to never be on either side of a murder in the alpha timeline (not counting when Jack kills his dream self).
Additional evidence for affection is its use in the form of moirallegiance with Gamzee in order to protect the other surviving trolls. Karkat's obtainment of "cahoots" with his session's Jack No