OK so about 6 months ago I dyes my hair almost black from blonde and now I want my hair to be blonde on top and black on bottom. I dyed my hair blonde but forgot to use a bleaching kit so I have blonde roots and dark brown hair. So I hae a couple questions
How long should I wait to bleach my hair?
Should I bleach all of it or just the part I want blonde?
After bleaching how long should I wait to dye it?
What products should I use to do the least damage to my hair?

Blonde do take a lot of time, especially when there is old colour still present.
Answering the questions in order
You can bleach your hair when ever but do not colour over the white sections or else you will get breakage you need to use bleach because you won't be able to get through the old colour.
U should do your roots every 6-8 weeks
Kerastase is the best products to use it is expensive but very concentraited so you will get ages out of the product. And will actually notice the difference it is and in salon product