Just you you all know not all blue crayfish will grow to six inches. Cherax destructor is from austrailia and is as far as I am aware the largest freshwater crayfish at about 8 inches. There normal color is acctually blue and not a genetic color morph. This species is one of the more common ones available in the US and the only one available for sale in the UK as all other species are listed as invasive there. Another popular variety available in the pet trade is the procambarus alleni which is raised on large farms in florida. The males stay a bit smaller but the females can get up to 6"
One of the less available ones in the trade is the orconectes immunis which only gets about 4". Most of these diffrent varieties I listed can be purchased off of the website I listed above. The owner of the site is a breader and other members also sell there stock on there.