Bleaching is very harsh, and to bleach black hair, you would have to do it several times to get it even and light, try a hair color remover before trying bleach, leave it on for longer than the box suggest, don't worry it wont damage hair if you get the right brand that says its not harsh, it may seem at first its not working, but give it time it will work, its not going to take all the color out, and it may come out uneven in color, but a bleaching after and then dyeing it a color to get it even everywhere will work fine. just condition like crazy, olive oil is good bc it helps repair the scalp.
Also you can use a color eraser. it comes in a box that looks like a dye box. you can buy it at drugstores. afterword beach slowly (in short sessions instead of all in one hit) then use a toner and deep condition.