You can get semi-permanent hair colours like Stargazer that would gradually fade. That comes in black, but I wouldn't guarantee it would come completely out of blonde hair. Whenever my naturally blonde sister used them they took forever to fade although they hardly lasted in mine (naturally dark, and used on bleached hair)
We dyed a blond male friends hair black when drunk as teenagers... 20 odd years later he still hasn't quite forgiven us for talking him into it - he stayed dark for years and then had to put up with being a horrible patchy dirty ginger colour whilst he grew it out. Alcohol is bad
To cut a rather long story short, I dyed my hair black about two years ago and I can't get rid of it. I've already had to cut my long hair short (it's like this now, sorry for the Katona picture!) after I had a bleach bath which did nothing but destroy the ends of my hair and make it horribly dry for about 3 months.
So I'm thinking the only way to get rid of it is grow is out. Unfortunately I have naturally blonde hair so roots are really obvious. I was wondering if temporary black hair dye actually exists, and if so if anyone could recommend one?
PS if you're considering dying your hair black DON'T