It's possible that the dark spots are spots of frostbite, especially if the foot got some water on it during a freeze. Also footpad dermatitis or pododermatitis, which is another name for bumblefoot, can look like that if a chicken has been living in wet soiled
bedding. If it is frostbite you can apply some iodine or antibiotic ointment, but if it is footpad dermatitis, you should soak the foot in epsom salts or betadine for several days in a row, and apply some antibiotic ointment such as Neosporin. That can be tricky with winter temperatures since they should be warm and dry afterward. Keep coop bedding clean and dry, and use pine shavings if you can. Straw and hay tend to hold in moisture.
Edited by Eggcessive - 2/6/16 at 7:16pm