if you are naturally blonde then see if you can get some spray hair colour in black or white and maybe green or purple – if you use anything to lighten it then avoid spray or hair dye as it will take weeks to come out (I have a friend who discovered this after colouring her hair orange for a fifth element costume and suffered peach hair for ages after 🙂 )
You could go with Black hair if you really want to shock your mates as it will make a drastic change, maybe keep a streak on either side blond? Probably wouldn’t fit so well with your gothic costume but one year I back combed my hair (very long and very straight) into a huge big bee hive around my head, sprayed it black then used talc to add two white flashes on either side. Took alot of pins and hairspray but the effect was impressive.
or go with white and put a few random skinny pleats either in green or purple through your hair – if you pleat them with a pipe cleaner thru the plait you can make them bendy or stand up a bit – like snakes?