He hadn't forgotten about the black band - hard to, really, seeing how prominently Spike's displaying it - but he hadn't reckoned on it actually being a problem here, considering he wasn't asking Spike to actually bite him, just suck up a bit of already spilled blood. Apparently, though, he was underestimating just how tasty a meal he is.
He yelps as he's pushed away from Spike and staggers back, almost tripping over his coat before he manages to catch his balance. "Oh, for fuck's sake!" he yells up at the sky, as if the AIs are gods residing up there in the heavens, as if they can hear him. (Maybe they can.) "I was trying to be nice! Can't you just let things work for us, just once???"