I know that if you try and grow carrots or parsnips in a clay soil the skin is stained?

Have they been waterlogged? Carrots also go black when frosted but we aint had no frost yet! Are they all black? Scorzonosa a salsify variant is black skinned.
I often get odd ones which are a very deep purple, bit like a black sheep in the family I suppose!
Not much help, I know, just thinking out loud I'm afraid.
PS A heavy infestation of carrot root fly will turn them black! Look out for little maggots in them.