this is really really reaching but i had a thought that through some paradoxical shenanigans thingamabobshdgskus or just the forces of nature deciding that yes it should be a thing, karkat's blood color might have dictated the blood color of the new species in the universe they created????? because let's face it he leaves his marks in many important parts of the story, primely caliborn and calliope's blood colors and text color so this. is also a thing that could happen?

and that would explain from our 4th (5th?) wall viewpoint why his blood color is the same as ours and not some other freaky color like black or white

I actually kind of thing is the opposite- With the heavy control Lord English has over Alternia, the fact Karkat’s caste are ‘Limebloods’, that were hunted to extinction, and that his Blood Color is Candy Red for being a mutant, tells me of LE’s influence over the society, and in turn, Karkat as sort of “THE Troll” to embody the group. Lime like Callie, Red like Caliborn, with his Grey being the middle point between them. There’s some pretty heavy symbolism there, and SPECIALLY when you consider the theory that his Denizen was motherfucking Abraxas.