what can be used to reduce the black color caused by rubbing legs?

Answer

First you need to make sure you are not rubbing them in the future. Using a little longer underwear can help. Blackish discoloration can be treated with Hydroquinone cream. In addition isotretinoin cream can used. You can even combine both. 

But for females isotretinoin is not recomended as it can cause birth defects in pregnancy.