At what temperature do your toes turn black through frostbite?

-15C or 5F when the blood vessels constrict, cut of the blood flow and the cells begin to die. This link will give you all the info you're after:

http://en.wikipedia.org/wiki/Frostbite

Think you already have a very good answer (your 1st response) and link provided.

However temperature obviously plays a part in at what point one can get frost bite, but I think mainly it is to do with poor circulation in the extremities, and at what point the circulation stops flowing would be different for each individual I guess.