You don't see what the big deal is. Nothing special. This kid's out of his mind.
But he's still blubbering on and on about it.
Author commentary: Karkat's blood color was concealed for only a few pages. Visually obscuring it until Jack looks at the knife is less about keeping the true color from the reader than about deliberately collapsing any mystery or controversy regarding the blood at the moment Jack looks at it, since he's acting as the witness who can verify for Karkat that his red blood is no big deal and is in fact perfectly ordinary to most creatures, including every living thing inside this game. This is another one of those non-reveal reveals, where the "mystery" surrounding something was never actually that important or even meant to be obscured, just like the ghost Aradia reveal. When you look back through the story, Karkat's blood color becomes obvious: his red hive decor, his Land, which surrounds him with HUGE OCEANS OF RED BLOOD. Almost like his environment is taking this point of extreme insecurity and rubbing it in his face. The mystery of his blood up to this point seems more fueled by Karkat's insecurity and fear rather than being a true point of narrative intrigue. The moment Jack shows him it's nothing to be ashamed of is when he can begin the process of letting go and dealing with it.