last week my hair was the colour of Dr. Pepper, really dark reddish brown, almost black. i went blonde in one night, and decided that i hated it. i used a colour remover (my hair is naturally light red brown, the dark was dye), then a bleaching system then a dye and toner. came out really gorgeous blonde...only come to find out that there's a reason i'm not a blonde anymore, it just doesn't look good on me haha. so i went back dark the same night.
it is of course, better to let a professional do this. my hair has stood up to about a million home dye jobs in the past two years and i use some really amazing products so that whole dark to light process didn't damage my hair AT ALL. BUT, that's not the case most of the time.
so i'd advise to make sure you want it platinum blonde before you start the process, because black to platinum blonde is a very long and expensive process, even if you're doing it at home. and it would be a shame to find out you don't really like it.