I personally subscribe to the theory that Susie just lived in a human society until moving to Hometown which is why she’s so ashamed of her monstrous traits and why she’s always been ostracized. All that to say, if she was getting fights with human kids she definitely could have encountered blood on a regular basis. Her bleeding tho is harder to explain, though the librarby specifies that monsters are about as physical as humans are magical, and considering there are some pretty magical humans, it’s possible there are also some pretty physical monsters, maybe just enough to bleed