hey there.
sorry that you had an oops.
i know going to see a colourist is pretty pricey, however, it would probably be your best option right now.
seeing as you have a gradient going on...(from what you've described, you had a dark/black colour, then a lighter colour, then your natural roots coming in?) the reason i suggest this is because if you try to bleach it further without knowing the chemical level of the product you're using, you could cause your hair to break, chemically.
(and it is most likely to break closer to the root, where it has stripped more colour [natural or otherwise])
if you are dead set on trying to fix it yourself, however, please wait at least, 3, but preferably a week between, with heavy conditioning between.
also, as someone mentioned before, put dye on dry hair that hasn't been washed for a least 24 hours, and start at your tips, working up to your roots. your timer should start when you have completed the application
feel free to pm me if you'd like more info or just to chat.
biggrin
best of luck!