Oh man, you certainly observed more things than I did. I feel ashamed at my obliviousness.
I too loved Sirius's endearments :) It was actually affectionate, like he really cared that something happened to Severus. There was Remus who Severus loved not doing a single thing, only caring about his personal sensations of smelling the blood and how good it felt. And there was Sirius, who although he hated Severus was horrified that Severus might die.