I would be saying that the bird was in fact a hen, going by its age. Ringnecks reach sexual maturity around 3 years of age and females can reach sexual maturity around 2 years of age. So if they don't have their ring by three years old most likely they never will. Blue mutation ringnecks 'rings' are different to a normal green.
Here is a picture of a male blue ringneck
A big congratulations to your friend Sky, ringnecks are great birds and kudos for giving a rescue a home. Would love to see some photo's of her new fid, I love perving on ringies there so pretty.