Maybe try to remove the purple/black before bleaching with color oops or something like that.
I'm afraid you'll have to bleach several times since the color you want needs a very pale base and henna is difficult to remove, you might fry your hair. So it can't hurt to try something less damaging before ^^
Anyway, i love the idea, good luck
ETA : oh and try this forum, i'm not a member there but they look helpful
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