If left long enough in the fridge it will spread into the meat in the carapace joint as well. I use a splash of vinegar in my water when I boil and it seems to reduce / delay the onset a bit. I think it is from excess moisture inside the shell. If you store them head down it also seems to reduce / delay the onset.

I will be interested to see what others do to avoid it?

my thoughts are this is the cray's blood, that's leaked into the joints.

anyone that tails crays too close to the body, or heads them fresh will know about their blood, that goes blue in colour and solidifies into a blue gel on the bottom of your esky. if left for a while or on the crays when you freeze them, goes black, and actually tends to make the meat go bad quicker.

all the above is in my experience.

 Tails.

Drowning in fresh water. Ice slurry kills them instantly. Never had black since doing this. Fresh water makes them swell big time. Must have something to do with black ? And Scotto is right blood.

can also happen if the crays are undercooked, I had the same issue a few weeks back. I left the crays on ice overnight and took them straight out of the ice and put them straight into the steamer, they came out undercooked and had black in the joints should have let them get more to room temp before I cooked them. 

Thanks for the advice.

 Boil for 15 minutes and they store for days with no black stuff. 

In my experience crays turning black doesnt affect the taste, just looks a bit wrong.

I find if you only just cook the cray they can turn black eg. boil 6-8mins for a legal cray, if you cook for longer eg. boil 10-12 mins for a legal cray they wont turn black.

I can't comment on putting crays in fresh water as i have never done this, always pack with ice or ice slurrey. I as others have mentioned i have suspected that the black colour is something to do with the blood. 

Never,never,never let any seafood see freshwater after you have caught it.
Despatch all in a ice slurry of salt water ,crays to be cooked in saltwater with cut up lemon (my pref).Take a 20 ltr container out and fill it with fresh saltwater just before get back to port or keep them alive until am ready to cook (over nighter at the rock.)
Crays can be half cooked cut in half cleaned patted dry and vac sealed for BBQ future use ,(no freshwater)
Fish:should be cleaned with salt water & then patted dry with a paper towel and vac sealed if you intend to keep (no freshwater at all)

Heads and guts from cleaning to be chopped up and kept for burley on next trip and put in the fishing freezer witch all good fishermen have.
Happy wife happy life.

(My views only)

 undercooked. need to boil the average size whites for 10mins (thats 10 mins actually boiling, and not comming up to boil), doesnt hurt to give them a couiple more minutes in any case as it doesnt seem to have any impact on eating quality.