The blue shell colour in all the breeds mentioned above has usually been bred in from an Araucana, and if you get a pure bred one you are guaranteed to get pure blue shells. There are different standards for Araucana; e.g. the US one is tailless, has ear tufts, and includes a lethal gene (so is not a popular option for that reason). The UK/ Australian ones have tails, do not have ear tufts, and do not have a lethal gene.
There's more info about it here https://en.wikipedia.org/wiki/Araucana
There's more info about it here https://en.wikipedia.org/wiki/Araucana