This is likely to be part of a scam. A note, dyed black with iodine (say) may be restored to its proper condition by the application of a ssd chemical solution and activation powder for sale of vitamin C.
Scammers present a bundle of black paper ‘notes’ and will clarify one of these as above. Next they are prepared to sell the ‘notes’to you at a good discount. This is a good time to get some walking exercise!