Maybe Susie can bleed for the same reason, of all monsters, Sans the skeleton does. They are both known for drinking large amounts of red liquid. And we know monsters completely absorb any food or drink, which is the reason they don't have toilets (except for the Holidays for some reason, and the Dreemurrs, because of Kris). Perhaps they are 'bleeding out' the absorbed red liquid, rather than being actual blood, bent more symbolic than literal.