for blood im pretty sure would give like buffs, i think it just generaly makes sense, karkat said it represented the warrior spirit or whatever that one time and it has to do with power through mass rather than individual, for an active class it would give very strong buffs to the hero and for passive it would give small ones to the entire party, the buffs maybe would have a vampirism effect were more damage on the enemy = bigger buff or something like that so the blood aspect grows stronger when blood is shed

for doom im not really sure, from its name you would assume it has something to do with doomed timelines but thats a bit to close to time even if it interacts with them in a different way or something, so another theory that might be true is it relating to death maybe like the more enemys they kill the stronger they are but thats basicaly the same as my theory for blood and they cant both do the same thing so i really dont know about this one

for rage i dont have many theories but what im thinking is maybe that they have like an ultimate bar in smash where if they take enough damage they can use their ultimate or something like pent up anger growing and growing, but i dont really know how that would vary across classes and again its basicaly just the same thing just in reverse

really blood is the only one i think is an actually solid theory out of my proposals but i thought i might come up for stuff for the other two anyway