How long is your hair right now and how much of your natural hair has already grown out? Would you be willing to cut it short in order to chop off all the dye? You say you coloured your hair a few months again so I'm assuming you probably have anywhere between 1-3 inches of new growth. In that case, if your hair grows at the average 1/2" per month, you're probably looking at waiting at least another year for it to grow out enough to even be cut into a bob, unless you want to get a pixie or something short like that.
I, personally, would be cautious about using bleach. I accidentally dyed my light brown hair black as well and didn't like it. I grew it out all the way to my nape (1 1/2 years) and then had the hairdresser carefully bleach only the dyed bits below. It was very damaging to my hair though and I am now dealing with split ends, breakage, and the bleach went very brassy. I wouldn't have done it at all and kept growing it out except that I had grad coming up in a month and I had already bought extensions in my natural colour so I needed to match them. If you are really miserable with the way it looks, then perhaps bleach is the best option for you. Also keep in mind that the black should fade over time and will probably become more of a dark brown.
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