So... Karkat hides is blood color from his troll friends throughout of the comic. Does he tell them after the session is said and done? I mean he is safe from the Alternian empire and all or would he be still suffer from it because that how he survived for so long?

Chances are they find out at some point during the Session? Karkat isn’t exactly good at keeping secrets, nor is Terezi. I would be surprised if Karkat ended the combat with the Black King with no bleeding wounds.