I think Mrs. Evil was on the right track. Maybe it's erotic to see someone messed up. I don't think blood is erotic, but there's always something sexy about the guy who takes a blow to the face and comes back up, wiping the blood from his lip. A tough guy thing. Maybe a tough-girl thing for you.
Hang on a sec. I'm a formerly huge x-Phile. Was it Gillian Anderson? Because Scully had thousands of bloody noses throughout the show, and even I, a straight girl, think she's hot.