Don't do that, You will totally ruin your hair!
Hair colour is like... a paint palette.
Imagine you've got black and you put yellow(blonde) on it.
It's not going to stay yellow is it?
What you need to do if you want to take your hair to a lighter colour is buy some hair stripper or get stripping done at the hairdressers - this will take most of the black dye out but will leave you with a ginger colour from the bleach in the black hair dye you've used. Then you gradually have to bleach over the ginger/red bits left in your hair.
Going to a light colour from a dark colour is expensive, takes a lot of time and damages your hair a lot.
Recently I stripped purple dye out of my hair and attempted to go blonde but there was too much red and orange dye left in my hair and after stripping and bleaching it, my hair was brittle and falling out!

So I just put on a brown dye on top of the light gingery and blonde as I felt it wasn't worth trying to go blonde.
If you are really serious about going blonde please consult a hairdresser or anyone with experience with this!