i've had this idea, and i'm not sure if its original or not. but it seems to me using black magic twists you because magic is what weaves reality and the 'firmament' together. to use it against its design opens the way for the decadent energy of outside to get in you. sorta like a backlash effect. was waiting to make a theory thread on it after i could explain it better. but to summarise, black magic is related to outside energy and this is why it twists you, indeed why the laws were made originally. (harry being starborn gives him his natural affinity for dark magic IMO) could be way off, but i believe there is a connection. also seems like only mortals can use black magic,(or should i say use magic that is considered black? sidhe do things considered black magic in mortal hands without actually twinging their magic 'color') could this be because only mortals have choice? same with opening a summon from beyond.(not counting the fraud vampire who i don't consider immortal enough)