It's extremely hard to get black dye out you could try using shampoo with sulfates but that might just make the black fade a little but it's really hard to get rid of black you would need to bleach it or strip it and then dye it blonde you can't dye it over black and you can't take black out without bleaching or stripping.
Well hair color washes out but it will take a long time and it probably wont go back to blonde. The only thing you can do is dye it blonde.
