I would suggest you not do that. Black is such a dominate color and the molecules that make up the color are big and not easy to shed from the hair. Black is a color you need to be very sure about because it's no coming back without a lot of damage. The hair will turn pink before it go blonde because you natural pigment will have to swell so big that it can absorb more light but that black will not totally go away. It will leave like a reddish orange color that will look pink when wet. Don't do it.