The top of the foot it looks like she has some small wounds too, have other chicks been picking at her feet or have you noticed her picking at her feet?
Investigate that black spot of the bottom of the foot further to determine if that's a scab or just poop/dirt.
Soak the feet in a warm epsom salts bath. Dry well, then apply triple antibiotic ointment to the wounds. You may want to wrap the foot to help keep dirt out of the scabs. Place her on clean dry bedding. Check the foot everyday.