I can't help wondering if maybe it's a sign of a painful past...I mean, they all seem to have one other than Sakura, right? Kurogane seems to strong and tough....but he's shown enough of him to let us know that he DOES let emotions get the better of him sometimes. I just wonder whether something happened when he was younger, and that's why he became so gruff....like maybe he was fighting to protect somebody, and failed, resulting in their death? Maybe he got the scar in that fight?
Hmm...it could tie in to his 'hatred' of people like Fai too....maybe the person who died was sort of like Fai, and the blonde brings back painful memories for Kuro-muu?