Greetings. Firstly - corns don't usually turn black unless there's an injury or you don't use foot wear all the time . the possibility of differential diagnosis of plantar warts has to be considered. If plantar warts - cryotherapy / radio frequency ablation are good forms of treatment. Corns can be treated by paring and switching over to suitable foot wear. Further queries may be sent on follow up messaging. See a qualified dermatologist near you. Warm regards