Extremely cold temperatures, wind chills, dampness, moisture, and more...all reasons that frostbite in chickens can occur.  The black spots you see on this chicken's comb is a mild case of frostbite. 
After doing research and talking to other chicken keepers, I think it's safe to say that cold climate chicken keepers will most likely deal with frostbite at one time or another.  This being my first experience, I did lots of research and want to share with you my findings for preventing and treating frostbite in chickens.  Click 
HERE to read more.