Hiya,
Im afraid tint will not lift tint, and also depending on how long you have been dying your hair black results will vary when having cleansing baths.
My advice would be to pop in for a complimentary consultation so that a colour specialist can assess your hair so they can advise you what to put on it causing minimum damage to the hair. With out seeing the colour and the build up we cannot recommend a product to use.
Good luck
The Team at MatthewDavid Bespoke Hairdressing
http://www.matthewdavid.co.uk