I would definitely do it gradually if at all, and bear in mind that blonde hair takes a LOT of upkeep. If your hair is naturally that dark you will get dark roots within a few weeks, so think about whether you want the expense/time commitment to keeping it looking good. You might also find that your hair loses some condition so I would recommend investing in a really good conditioner and have regular treatments.
For what it's worth, if I had your colour hair I definitely wouldn't do it but mine is muddly blonde and looks rubbish without highlights, so I am a slave to the bleach.