Lemon juice won't do much by way of lightening hair that's been dyed black but it will lift a teeny tiny bit. Lemon juice is also a great natural hair conditioner because, like vinegar, it is acidic ;-) There's nothing to lose by giving it a try ;-)
You can try peroxide but when it lifts it will have a brassy tone so if you do it be sure to experiment first with some hair clippings in a clear glass bowl to make sure you'll like the final effect. You can check every five minutes to see the lightening effects/progress.
Use straight proxide, might have to do it several times, using a hair dryer to heat so it will activate it. but after doing so use a good conditioner or your hair will be extremely dry.
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