As much as I've been squee!ing about necksucking, I think Fai will drink it from Kuro's wrist. That's the feeling I get, but since I don't have anything to back it up, I could very well be wrong. Clamp's been too nice to us lately, I'm reluctant to believe they'd be nice enough to give us Fai actually sucking on Kuro's neck. *sob*
And I agree with Fai being reluctant at first. It is, in a way, hurting Kurogane and Fai doesn't want to hurt anyone. And who would really jump right in to sucking someone's blood? Even if it's instinct now for him to want blood, he probably won't accept it just like that.