How do I go from Blonde to black and back again?! :mad:

I wouldn't, if I could avoid it. Even if you use a temporary dye to make it black, it will most likely never fade entirely from your hair. Once black, you will need to strip it to go back to blonde, and it will be difficult to get the same blonde as that which you started with. Your hair will most likely be heavily damaged from dying, stripping, and dying it in quick succession.