So, I really don’t know why this hasn’t been pointed out yet. When the Blood symbol was first revealed, I noticed the parallel but didn’t consider the implications.
All throughout Homestuck, symbols have had more importance than just representing a character. They are a sort of motif that represents something of a larger scale — Tavros’ and Vriska’s symbols have been used to represent the powers of mind control over animals and people, respectively. Meanwhile, the god tier symbols appear often, especially when someone is using the power it represents.
Personally, I feel like this is evidence pointing to the fact that the aspect of Blood is one that holds a mastery over the connections between people - it’s an element of brotherhood, of peaceful negotiations. Slick cut his hand to show Karkat his blood and formed a bond with him — they were in cahoooooooots. Karkat’s role could be to protect the brotherhood that keeps the trolls alive, which is suggested by his pacification of Gamzee. Karkat has always been adept at understanding romance and emotions. Perhaps his role as Knight of Blood is there to complement his strengths but challenge his own doubts about his ability to protect his friends.