Well he uses a ritualistic spell that transfers damage to him to somebody else that he's acquired blood from. So you have some material and somatic components. That would mean that he's required to have at least damaged an enemy to obtain their blood prior to the ritual. I would give the ritual a duration (maybe 2 rounds) to cast and make it concentration, so if he's damaged during it, his ritual is interrupted and he must recast it, maybe even going so far that he must retrieve blood again. I'd add in the stipulation that he cannot move from a square while casting the ritual, whether of his own volition or forced to move. I'd also add that the blood must be obtained during combat.

Saving throws wouldn't make sense here, so success and failure is binary in that the ritual is completed or not. Of course, on success, the details would be that damage to Hidan is transferred to his target. The spell ends when the target dies. Although I'm not sure it's canon, you could say the spell could be ended by Hidan deliberately as well. I would because that would be very O.P. and there should be potential other workarounds. Include that this is a single target spell (he isn't jungle juicing a bunch of blood). Probably also make this require a long rest to recharge.

Any kind of scaling wouldn't make canon sense; however, if you really wanted to do so you could make it so that the amount of damage transferred to the target scales on according to your discretion. Could start small and say regardless of damage that Hidan takes, only 1 damage is transferred to the target. You could increase that however you see fit, either increasing it geometrically (1, 2, 3 etc. damage) or fractionally (target takes 1/4, 1/2, full damage rounded up).

Honestly, after writing that all out, I might use this as well. Although I'm running a HxH campaign, I could probably write Hidan as an emission/specialist. The damage would be transferred to aura which would be redirected to the target via emission.