Its very hard to go from pitch black to blond quickly without messing up your hair (if you have long ones , you don't want to do that). I did the same thing, off black (my natural color is off black or darkest brown, not quite sure which), and it took me 6 months to be light again (level 7 red with level 8 copper blond highights, level 7 is usually considered (medium), level 9 (light)), I went slowly.
Did you do it already? Wouldn't it have been better to go for a warm rich darker brown (level 4 maybe). Then, going back to blond is much quicker, easier and less risky.